It's very simple. Basically you want to make just one trip to Young Emmett's lab. The last time you have no choice so it don't count. Your first time there you have to change out his mind map card, get the cleaning solution, get the picture album, Do everything you can there. Then you should have nothing left but to go and get the solution aged in the Delorean and to give Trixie the picture album.
After all that you'll go back one last time automatically due to storyline requirement. You won't have the trophy pop up however till after Edna dumps him and he drives off for the final section of the game.