Though most often used for furniture, biscuit joints have many other applications. The joiner makes precise plunge cuts into each side of two pieces of wood that are butting up together. Then a "biscuit" (a wafer of compressed wood chips) smeared with glue is inserted with one side in each slot (see slide #3 to learn how).
Biscuit Sizes: Biscuits are commonly manufactured in three sizes for different thicknesses of board- 5, 10, and 20.
Whatever you are making, you need to judge how many biscuit joints you need for a good connection. Thickness of the biscuit should be judged on wood depth. Butt the wood together, aligned where they will be joined then draw a pencil line across the two pieces to mark the biscuit joint's position.
Push the other end of the biscuit into the corresponding slot to join the two sections and clamp. Use a damp cloth to wipe away any excess glue.
If you end up with a lot of glue squeeze out, it may be helpful to let it cure for 30 min. or so, then scrape of the slightly hardened glue.