Marvel's Guardians of the Galaxy

There are 15 Guardian Collectibles in the game, scattered across Chapters 1 through 13. You do NOT need to collect all of them for any achievements, so don't fret if you miss any. You need to collect a total of 8 of them to unlock Thoughtful Captain (15G) and then all of a specific character's collectibles for this achievement.

Unfortunately, there is no obvious in-game way to see how many collectibles a specific Guardian has available, and the only way to see which ones you've found is to go into their bedrooms on your ship and look around for them. However, here is a quick breakdown of all of the collectibles and which chapter they're in.

Quill
- Ravager Comm Bracelet (Chapter 1)
- Chitauri Sear-Cuffs (Chapter 3)
- Galactic War Commendation (Chapter 7)

Gamora
- Assassin's Ring (Chapter 4)
- Collector's Emporium Doll (Chapter 6) (costs 2000 units and can potentially void you from Galactic Frugality (15G))
- Meditation Stone (Chapter 10)

Drax
- Drax's Rap Sheet (Chapter 7)
- Mourning Candle (Chapter 10)
- Xatar's Blade (Chapter 13) 

Groot
- Broken Translation Device (Chapter 2)
- Cold Case File (Chapter 5)
- Deeproot Plant (Chapter 13)

Rocket
- Spinal Control Unit (Chapter 1)
- Deep Mine Device (Chapter 8)
- Glarnot Mask (Chapter 12)

As you can quickly see from this breakdown, if you follow along with the guide below, by Chapter 7 you will have collected all of Quill's Guardian Collectibles and will unlock this achievement. You will also have collected eight Guardian Collectibles by the end of Chapter 7 and will have unlocked Thoughtful Captain (15G). 

WARNING: I highly recommend skipping the Collector's Emporium Doll in Chapter 6 in the guide below. It costs 2000 units, and you need to be sure you have enough units left to pay your fine to Nova Corps in Chapter 9 to unlock Galactic Frugality (15G). It's best to skip that collectible to be safe.

Once you've unlocked both this achievement and Thoughtful Captain (15G) (by the end of Chapter 7, if you follow along with the guide below), you can ignore all Guardian Collectibles for the remainder of the game, if you wish. There are no achievements related to collecting all of them, or any more beyond what you've collected for those two achievements. 

The following video guide shows all Archives, Items, and Guardian Collectibles in the game in chronological order. NOTE that even though the choice you make in Chapter 3 significantly changes your path in Chapter 4, the Guardian Collectible is present in both instances in Chapter 4, but in different locations obviously. It isn't missable nor do you need to replay Chapters 3 and 4 to obtain it. Just watch the section of the video for the path you chose.

Video courtesy of MrWilliamThor: