Fir this trophy you need to collect the 4 items of clothing in the game. You cannot get this trophy before you finish Act 2.
Knit Cap: In the mailbox outside Strong Bad's house.
Data Boys shirt: From Bubs' Stand go left or right until you reach the Photo Booth. To the right of the photo booth is a cardboard box with the shirt underneath it.
Moustache: After you win the race go to The Track and go right until you find a microphone. Use it to see a cutscene of something falling out of a speaker. Go back to the starting line to find the moustache on the ground.
Red Chef's Hat: You can find this on top of the microwave in Act 2 after you get Homestar to leave the kitchen. I have written a quick guide on how to do this in the spoiler tags below.
Please vote positively if this guide was helpful and feel free to leave a comment if I can improve it in any way.