A plate joiner is a tool with one specific purpose; to make small slots in pieces of wood so that a biscuit, or join, can be used to hold two pieces of wood together. Using a plate joiner takes much of the worry out of making these slots, as hand-held tools can be dangerous.
Step 1 - Cut the Slot
The first step is to make the cuts that you need in the wood. Line up your two boards, so that they fit closely together. Put them into the joined position, and then make several marks in the boards to show where the biscuit slots will have to be cut. Taking the first piece of board, put up the guide fence, and line this up with the first mark. Put the blade into the wood. Repeat this for each slot on both boards.
Step 2 - Performing Angled Cuts
A modern plate joiner will also have a bevel saw which will allow you to cut your biscuit slots at 45 degree angles. This follows the same principles described above. Mark out your slots by positioning the boards where you want. You can then cut your biscuit slot along the angle which you want the joint to have.