My students did fairly well one chunk at a time, but once it came down to not only remembering which category you were in (avoir, VANDERTRAMPP or reflexive) but what pieces you need for each category, things started getting messy. They were adding reflexive pronouns to VANDERTRAMPPs, agreement to avoir, it was just a huge mess.
To help them work through this, I decided to take them back to the building blocks of each type, quite literally. Courtesy of my son, each group was given big plastic legos. I labeled the legos in advance and each group received: subject, avoir/etre, reflexive pronoun, past participle and agreement. I used tape on each one so I could write those categories in permanent marker on the legos without ruining them. Then, around the room, I put cards with a subject and a verb. Their task was twofold. First, they had to figure out which category of verb it was in the passe compose. Then, they had to get the right lego pieces and write the corresponding word on the lego piece with dry erase marker (this works awesome..wipes right off). Once they did this, then they could write out their full answer on the sheet by putting the pieces together. They could check their answer by lifting the card. The answer revealed was the "pieces" they would have done as well as the full written out answer.
It seemed silly to me, but they really enjoyed it. It got them thinking about the different parts of forming the past tense depending on category and it helped them to visualize how it all goes together.
In hindsight, I wish I had done it as we learned each type as well, not just when putting all three together. Definitely something to do next year!