Line up the pieces of wood to be joined, with their grains facing the same way. Make sure the tree rings on the edges of the wood are in the shape of an arch, with the open end of the curves facing downward. This positioning will prevent the wood from curving when humidity is high.
Make pencil marks along each joint where you want to put a biscuit (Image 1). The biscuits should be about 6" apart. Spread the boards apart. Using a biscuit joiner, cut a slot at each mark, making sure to center the cuts according to the marks (Image 2). Apply a little bit of wood glue along the length of the edge to be joined as well as inside the slots.
For perpendicular joints, such as in the construction of bookshelves where the edge of one board is fastened to the face of another, hold the boards to be joined together and make pencil marks on both boards to indicate where the biscuit cuts should be made. Space the cuts about 6" apart. Place the two boards parallel on top of each other, with the top of the horizontal board aligned with the line you just drew. Clamp the two boards together in this position.
Use the biscuit joiner to cut slots in the end of the top piece of wood. Turn the joiner to a vertical position, and cut matching slots in the bottom piece of wood. Assemble with biscuits and a small amount of wood glue. Clamp the joint together and allow the glue to dry.