After the 6th PC, you're left with 3 pieces from a bag. Make a setup based on these 3 pieces.
100% Setups
Due to the nature of 7th PC, most of the setups are 100%. four.lol has a guide containing all 100% 7th PC patterns.
Non 100% Setups
If you have S and Z for 7th PC, you can't do a 100% set up (given that the other piece isn't T). This means that S+Z+I, S+Z+O, and S+Z+J/L don't have 100% setups.