I rebatch 95% of all my soap. You can do so many neat things with rebatching that you can't do before your soap is saponified. These are basic instructions, and the recipes I've thrown together quickly can be found here . It's not hard to come up with your own recipes either.
Okay, first off, I'm going to assume you're using 4 cups of grated soap, and you can use more after you get the hang of it, but until you do don't use a whole lot of soap because it's better to ruin a small batch than a large one! Don't expect it to be perfect every time.
First off, grate your soap down to thin little shreds. The older it is the harder it will be to grate. So do it while the soap is still fairly green. It will also melt quicker and smoother if it's fresh. After you've done this and you have around 4 cups, get a bowl you don't care to ruin. Pour 1/2 cup( or a full cup if the soap is old and crumbly) of milk into the bottom, and then pour the soap on top of that. Mix it up well, so all of the excess milk is absorbed with the soap. Let it set about 30 minutes, and then take it to the microwave. Zap it for about 3 minutes, but watch it. It has a tendency to foam and you have to stir it, especially when it starts getting hotter. After the three minutes, melt it in 30 minute intervals until it has a smooth almost transparent texure and color. You can rarely ever get ALL the little tiny pieces of shred melted. This is the time to add color, oils, moisturizers, or fragrances. Then simply pour into the mold. If your soap is too lumpy, or if its too thick and wont pour then add a little bit more water or milk. It can always evaporate out. It's that easy.
You can also melt soap on the stove or in the oven, but it takes much longer and I don't like to do it. It's best to just go with the microwave!