For stuffed shells, we used jumbo shells since that's about the only thing you can stuff by hand. Do not precook the shells. Stuff them when they're cool and dry. You should use a larger slow cooker for this so you have more surface area for the layers. Depending on the shell, you will probably have to try a few things:
- Scoop some cheese with any open end of the shell.
- Cram some filling into the shell using a spoonful of cheese.
- Push the cheese in by hand, using your pinky finger to penetrate shells that are more shut.
Use some sauce at the bottom of the pot to reduce the risk of the shells sticking to the pot. You can lay them down any which way you prefer, but we used mouth-side-down. You can safely make two rough layers of shells, stacking the second layer on the first where the nooks and crannies allow. If you get too ambitious stacking shells, the bottom shells will start getting crushed by the shells above it, which takes them out of the fun shape you're trying to make.
Top the shells with some more sauce. They do not need to swim in a pool of sauce. Sprinkle on some cheese topping and cook on low for 2 to 3 hours or on high for 60-90 minutes. Check how done they were by trying to pierce them easily with a fork. We have found cooking on low has turned out better shells.