The First Descendant

Warning: As mentioned in the Road Map, many of the cumulative achievements in this game are finnicky, so I would suggest doing this all in succession, or at the very least in a single play session to ensure it unlocks properly. The tracking may reset if you log out of the game. This is especially important for this achievement due to the time it takes to get all the crafting materials!! All of the other cumulative achievements can be done very quickly, but not this one.

Step 0: Buy 10 Crystallization Catalysts
If you'd rather avoid the lengthy process outlined in the next step, you can just purchase these with real money as mentioned in the Road Map. You'll still need to do steps two and three, but those are the least difficult part of this achievement by far.

Step 1: Research 10 Crystallization Catalysts
This achievement is the main reason the game takes as long as it does, if you're planning to play for free. To assign a module socket type, you need a Crystallization Catalyst item. These are researched with Anais on the right side of Albion from the spawn. To research all ten of these, you need the following:

It's also important to note each one takes 7.5 hours to research, and you can only one at a time, so if you grind all the materials first you'll need 75 hours of research time before you can use all ten - make sure you rotate the materials and get enough for one, then research it, and repeat. The easiest guide I've found to follow on obtaining all these materials can be found HERE, which shows where each one drops, how to quickly reset whatever enemy drops them, and what difficulty to play on for the best drop chances.

Now, with all that said you will have the opportunity to receive a few free Catalysts which should allow you to lower the total number of materials you need to farm. These are as follows:

In addition to these guaranteed ones, you also have a chance to receive a fully crafted one if you can complete all 21 waves of the Echo Swamp Albion defense mission. However, most people tend to quite after Wave 7 or 10, so it's unlikely you'll be able to farm these, especially when it's not a guaranteed drop.
 

Step 2: 10 Level 40 Weapons/Characters
Now, to actually use the Crystallization Catalyst, you must have either a character or weapon at Level 40. Since you need to unlock a minimum of five Descendants for Descendants, Assemble (40G), that's half the battle right there. While you are leveling up your Descendants (and likely collecting all the materials in Step 1), be sure to swap out your primary weapon each time it hits Level 40, and keep the same secondary weapons equipped the entire time. Your secondary weapons receive partial XP for everything you do, so they will level up much more slowly. See What Was Always Expected (40G) for the best experience farming missions.
 

Now you can actually do what the achievement asks! However, as warned in the Road Map and at the top of this description, you want to hold on to all ten of your Crystallization Catalysts until you have all ten characters/weapons available and assign all ten module socket types right in a row to make sure you get the achievement. Go to the module screen on each weapon/character and choose the "Assign Module Socket Type" and spend the Crystallization Catalyst.

Note: This process will reset your character or weapon to Level 1, which is another reason you'll want to get all ten characters and weapons to Level 40 at the same time.