Using a very sharp exacto knift, cuts are made away from your body and hands. Many cuts are needed to get all the excess foam cut, so don't try to do it all at once. More numerous shallow cuts are easier to control.
Keep a firm grip on the foam. Choose the highest density type of foam you can. Be careful not to get deeper gouges on the sides or knicks (from knife, fingernails) on the top as you'll see those impressions with every piece you make from the mold.
After you get it reasonably close to the shape you want, use a carpentry shaver to smooth it down. Have a shop vac handy to clean up all the shredded foam.
Use sandpaper (we used a medium grit) to smooth it down. This is where you want to be extra careful about nicks. It's easy to sandpaper smooth it out, but the foam can shred and leave holes or depressions if you try to rush it. If you want a curved edge, you can try to wrap your sandpaper around something curved instead of wooden blocks as we did.
Here's a small sample of the hump-style molds we made.
You'll want to protect the edges, but be aware that the edge line from the tape will form an impression in the mold, so try to line it up with edges already there in the mole. We used duct tape but other types are fine.
The tape will stick better if you get the fine dust off. With all the sanding, they had a good electrostatic charge, either vacuum or wash them.
An inverted piece can also be made. A line about halfway on the inside provides that cut line to form an even angle. If you choose a wider angle, you'll want to leave the clay in the form longer to stiffen as it will be more likely to break under its own weight when removed.
Once you get the desired slope, smooth off the edge as in the previous example.
Roll out some clay, press it over the mold and cut off the excess around the edges. With this type of mold, you can use stamps to give patterns in the clay.
Edges can be smoothed out now, but it is easier to wait until the clays has stiffened up to avoid distorting the shape.
Keep using the molds until they get damaged. Have fun!