Place a slice of sandwich bread on a cutting board, and use the outer ring of the sandwich sealer to cut a round circle from the middle of the slice.
Repeat with the remaining slices until you have 12 rounds.
Repurpose the bread remnants by making them into breadcrumbs, croutons, etc.
Spread each round with a thin layer of peanut butter (approx. 1 ½ teaspoons), leaving a ¼” edge as margin.
Add a small amount of jelly (or jam) to 6 of the sandwich rounds, spreading it into an even layer on top of the peanut butter (and maintaining the ¼” edge).
Pair each peanut butter and jelly round with a plain peanut butter one.
Fit the round sandwich into the outer ring, then use the inner ring to press down and seal the two slices together.
Repeat with the remaining sandwiches.
To freeze: Place the sealed sandwiches on a parchment-lined baking sheet and place them in the freezer. Freeze for 2-3 hours, or until frozen solid. Then transfer to freezer bags. Label, remove excess air, and seal.
To serve: thaw any frozen sandwiches at room temperature for 30-60 minutes, or add them directly to a lunch container - they will thaw by lunchtime.
Notes
Storage: Homemade Uncrustables can be stored in the freezer for up to 2 months. For the best results, wrap the sandwiches individually in plastic wrap before placing them in the freezer bag. Without a sandwich sealer/maker: Use a wide-mouth canning ring to cut the bread to size, then flip it upside down to seal the two sides together. Alternatively, you can cut the rounds with a biscuit or cookie cutter, then pinch the edges of the bread with the tines of a fork to seal.