Start off with a larger area using puppy pads and let your bunny choose a corner to use. Put some hay over this toilet area to encourage them to use it and mix in a bit of the litter you are going to use. Always leave a bit of mess each time you have a clean out so your bunny can find it again more easily. Gradually reduce the size of the area then swap it over to a litter tray. It is important to clean up any mass from accidents elsewhere. The smell can cause a habit to form in a new area.