Neverwinter

Forgive the common sense answer here, but you must be in a guild to receive guild marks. Once you are in a guild, go to your guild stronghold (simply click on any zone exit and select "Guild Stronghold" from the menu. Once in the Stronghold zone, enter your stronghold and in the back left you will see a Mimic chest. Click on this chest, it is your guild coffer. By donating items to the guild coffer, you earn valuable points toward upgrades to your guild's stronghold as well as earning guild marks when you donate. The amount of marks earned varies depending on what item/quantity you donate. With a Level 1 Guild, each character can earn a max total of 30,000 guild marks to start, so you will need to max that on 2 characters to obtain this achievement. The 60,000 is accumulative across all characters it seems, so if you earn 10,000 on 6 different characters, the achievement should pop.

There are many different things you can donate to the guild coffer to earn marks....

*Labor = Donate crafting professionals to earn this. The higher the grade, the better value you will earn
*Gold = Donate in increments of 10,000 copper. (Not recommended, only earn 1 mark per gold spent)
*Glory = Donate Glory earned through PVP.
*Surplus Gear: You can donate gear you loot as well, be sure to identify it if you need to. (Green, Blue, Purple, Orange and I assume Teal for Mythic works)
*Gems: Donate Enchantments or Runestones, the higher the rank the more you earn.
*Astral Diamond Chests: Simply donate Astral Diamonds, 1,000 Diamonds = 1 chest. (Not recommended, only 1 guild mark per 1,000 Diamonds)

You can also earn Guild Marks by donating Campaign Quest Items.

Treasures of Tyranny = Donate Pages, Scrolls, Sigils, Dragon Fangs and such from Dragon Campaign.
Fey Trinkets = Donate Crescents, Sparks, Seedlings, Blades and Charms from the Sharandar Campaign.
Dark Gifts = Donate the Scrolls, Books and such from Dread Ring Campaign for this.
Frozen Treasures = Donate the Symbols, Dwarven Coins and Tears for this from Icewind Dale Campaign.

There are also other rewards you can loot off enemies in the Stronghold Zone, from Heroic Encounters in the Stronghold, from PVP matches or obtain from the quest givers in zone that you can turn in towards Stronghold Upgrades and Guild Marks as well. Completing dungeon quests from The Cleric reward Dungeoneer Shards. Completing quests from "The Builder" in your stronghold reward Heroic Shards. The Ranger gives Heroic Encounter quests that reward Adventurer Shards. PVP Quests reward Conqueror Shards. Whether you get them from an encounter reward, quest, PVP or simply as a drop off a mob in the Stronghold Zone, simply go to your guild coffer and click on it to donate.

For those in smaller guilds, who play on off hours or simply prefer to solo your adventures this is my recommendation. Start by doing all your initial quests in the stronghold. Once you finish the quest from the Ranger for killing 100 mobs in the Stonghold, come back the next day and you will be offered 2 quests. 1 for killing 100 mobs again which rewards only 1 enchantment, but the second is for killing 3 heroic encounters. (Soloable encounters are the Spider Attack, Drake Attack, Beast Attack and Devil attack. Others can be solo'd, but not near as reliably. I personally hated doing a Supply Run, spending 13 minutes only to run out of time fighting the final wave). Finishing the 3 heroic encounters rewards 10 Adventure Shards which = 100 Marks. You also get rewarded "Influence" which is turned in to the coffer. 150 Influence for first encounter, 120 for second, 75 for third, 35 for 4th and 20 for 5th. Then talk to the Builder for his daily quest which will reward 10 Heroic shards you can turn in for 100 guild marks. So simply by doing 5 heroic encounters including the 2 quests from the Ranger and 1 from the Builder, you can earn 600 guild marks daily per character. You should also obtain a few vouchers/tokens to turn in as well that count towards Labor, Gold, Astral Diamond Chests, the various Shards, Campaign Items, ect.

I Solo'd my 2800 Haste Cleric and my 2100 Bubble Paladin each night until I finished. This will be much easier in an active guild. Simply group up for Heroics, finish up the Daily quests in quick groups and run the Dungeons as they come up. Typically the quest givers rotate quests every 8-16 hours in the Stronghold.

*Note-1* I removed my Wizard from her guild just to see if the Guild Marks remained, and they carried over even after leaving a guild.
*Note-2* All of this information was obtained with level 70 characters. Please note that some quests and rewards might be different based upon level. Thanks to "thepack1221" for this information.
*Note-3* Once your guild gets a Marketplace and ranks up, you can spend the Guild Marks you have earned so that more can be obtained. This means this achievement is attainable with a single character in the right Guild. Thanks to "Tommy2Shotz" for this information.

Feel free to leave any comments or suggestions to this. I am not sure what all you get from the Dragonflight rewards, so any extra info given I can add to this solution. Unfortunately if I can't solo it, I have not killed it yet in the Stronghold Zone.

I recently got this achievement the 'easy' way. I didn't spend time collecting anything other than labor. The only other currency to donate that at the time was somewhat worthwhile were the Heroic Shards, Adventurer Shards, etc, you get from doing Stronghold Heroics(those Shards are worth 10 Guild Marks each).

However, that was really slow and took more time that I wanted so here's what I did:

1. I went to the market and bought as many black Ice Crafting Masters and Grandmasters as I could.
(You only need 20 Grandmasters and 20 Masters OR 120 Masters to most efficiently fill the Guild marks.
Grandmasters are worth 2,500 Guild marks, and Masters are worth 500 Guild marks. Common colored people are only worth 100 Guild marks, and white are worth 10 so definitely not worth using!)
2. At the time I did it, I was able to buy all the Crafting people I needed for around 30,000 Astral Diamonds.
(That will vary, but since Black Ice Crafting was currently inactive, those were really cheap. I was able to get some Grandmasters for less than 1000 Diamonds, and Masters for less than 300 Diamonds.)
(It will be different for everyone else, but how I searched for these crafting people was this:
Go to the Market, in the top box select "Professions", search box put "Master", then search. Switch the search feed to "Buyout". Then you can see the cheapest current blue people for sale. Next switch the Rarity to "Purple" to see the cheapest Grandmasters.)
3. I used the first method detailed below, the "Random Guild Method" to get my Guild Marks.


Note: A new guild maxes at 16,000 labor, so the best is to simply contribute one Grandmaster and one Master to the guild for 3,000 guild marks.
Note: To donate any currency to a guild, you most go to the Guild Stronghold map, cross the bridge, go slightly left into the Stronghold and straight to the back wall. There is a raised platform at the center of which is a Mimic; if you press on the mimic, you can see all the currencies that you can donate.
Note: Characters must be at least level 15, not in another guild, and in the same map instance of Protector's Enclave to create guilds!
Note: To make a guild, you talk to the Guild Registrar close to Knox in Protector's Enclave.


The Random Guild Method:
1. Once you have enough Masters and Grandmasters to donate, keep an eye out whenever in Protector's Enclave for someone saying something like "Need help making new guild!"
2. Help them make the guild by joining their group.
3. Once the guild is made, quickly donate a Grandmaster and Master for 3,000 Guild Marks(sometimes get paid a little something for helping)
4. Leave the guild!
These Random Guilds were very helpful and happened most often on the weekends.
I used this method for the majority of my Guild Marks! You only need to join/donate 20 guilds total for this! Over time this worked very well for me.


The Guild Spam Boosting Method:
If you want to get this done faster, there's this 2nd option:
This second method is the boosting method which I never actually tried before I finished the achievement but is really really easy, just takes a little while and requires the host to be organized.

Setup Phase:
1. You need at a minimum 5 people including yourself. I recommend going up to 8 people in the session to speed things up, 8 because that's the max? party size on the One currently.
2. Make sure everyone has their crafting people ready to donate.
3. Everyone add each other on friends lists.
4. Everyone has two characters ready to go if necessary(one character can only hold 30,000 Guild Marks and it doesn't count across characters)
5. Everyone meets in the same instance of Protector's Enclave.

Boosting Phase:
1. Host determines order and everyone remembers who they are behind.
2. Host begins by inviting 4 people, making a guild, then immediately kicking all 4 helpers from the guild, rushes to the Guild Stronghold, and starts donating crafting people.
3. Right after the host has make his guild, person #2 invites 4 into his group, makes a guild, kicks everyone from the guild, and goes to his Stronghold and starts donating.

If you time it right you can constantly have 2-3 people donating. The bottleneck will simply be the people getting to the Stronghold and getting back!
Very important: whenever someone is leaving the Stronghold to go back to Protector's Enclave, you must invite someone in the session who is in the correct instance of Protector's Enclave into your group otherwise you will most likely end up in the wrong instance of Protector's Enclave and have to wait 2 minutes to change.