1. Smooth the slab on both sides.
2. Cut the slab to the right size and shape.  It can be helpful to use a template.
3. Position the slab over or in the mold.
4. Define the angles, sides and rim with a rubber rib and a sponge.
5. Finish the rim with a bevel tool, or as you wish.  You might add an extruded rim.
6. Add handles if desired.

Soften any sharp edges of your plaster mold with a fettling knife.

As a general rule, increase the thickness of your slabs as the mold size increases.

A textured slab can add interest to your finished piece.  Experiment with such things as heavily embossed wallpaper, lace, rubber mats, or gunnysack.

If form pulls up in center while drying, slow down the drying by covering with plastic.  

Diminishing the Depth of your Slump Mold

While it is obvious how to change the depth of the Hump side of the mold (add more or less plaster), it is not so obvious with the Slump side.  The answer is to do two pours.  

1. Coat the mold with mold soap.
2. Pour plaster into the mold to the level you desire.  (This will go into the "ring" or "foot" of the slump mold, and will fill the depth that you don't want.  For example, if you want the mold to be half the maximum height, fill half the "foot" of the mold with plaster.)
3. Shake mold to level plaster.  Let set up.
4. Coat this plaster three times with mold soap, letting the soap dry between coats.
5. Fill mold to the top with plaster.
6. Separate the two plaster forms.  You will throw away the first pour, and be left with the second pour which is less deep than a completely filled mold.

Another way is to put clay in the "foot" of the mold, smooth with a rib on a banding wheel, and pour the plaster. 

Changing the Contour of your Molds

Hump Molds: You can use clay to fill part of the mold, then pour the plaster around the clay.  For example you could use clay to block out the corners.  Or stick clay coils to the bottom in a design, which will be molded into the plaster when poured.

Slump Molds: You may want to change the contour of the wall.  Here is a series of steps used to modify an Oval Slump mold by reducing the depth of the wall and changing its contour.