You must complete 4 missions from Tannis, who is located right where you start in Tartarus Station. For these missions, you must collect rebel claptrap parts, which drop after killing a claptrap.The missions (and part required to complete them) are as follows:
Fight For Your Right To Part-E (collect 42 parts)
Parts Is Parts (collect 75 parts)
We All Have Our Part To Play (collect 100 parts)
A Part Of Something Larger Than Yourself (collect 150 parts)
Do not waste time collecting more parts once you have completed one. For example, if you have collected 42 parts for Fight for Your Right to Part-E and continue to collect parts before turning it in and starting Parts Is Parts, the extra parts you picked up will not count towards the next mission. So be sure to stop collecting parts once a mission is complete and ready to "turn in."
The two best places to find claptraps for their parts are in the factory in the Dividing Faults area and farming MINAC (final boss in DLC). Though the factory is a good place to farm claptraps, farming MINAC is the better route since claptraps continue to respawn endlessly until you defeat MINAC. Additionally, this will help go towards obtaining other trophies in the DLC related to claptrap item drops.
Once you've completed all 4 missions and turned them in to Tannis, the trophy will unlock.
Happy Hunting :)