You’ll gain access to User Generated Content after completing the story mission “Karma’s a Bitch", which, although it counts as a side mission, is needed to progress the story of the game. UGC missions show up on the map as a green "i" icon. They vary but most are relatively short and simple. To start you should look at the "Trail Blazer" trophy and get that out of the way for your first 5 levels, then it would probably be best to switch your mission filter to either Featured or Famous missions which will be of a higher quality since, as with most UGC systems, there is a lot of crap out there.
For the purpose of this trophy, you may wish to repeat the same UGC mission over and over again, by saving it to your favorites. You will however only get experience from your first time completing them. Completing one a second time or more will not earn you experience, so unless you don't need the experience for your final few upgrades, it is more efficient to only play each mission once.
- User-Generated Content Mission.