A very quick description of a one-piece matrix mold is as follows. I'll be using a face as the example of what you'd be molding.
1. Here's what I want to make a silicone matrix mold of.
2. I recommend sealing a clay sculpture with Crystal Clear first. Then cover the piece with paper towels and wet them down, with a spritzer, so they conform to the sculpture. don't soak them, just get 'em wet enough to settle into the shape below. You can then put plastic wrap over it if you feel the need. I usually don't as the weight of the clay matrix can push the plastic into the clay sculpture and leave harsh lines in it. You may get some paper towel texture on your sculpture but that's pretty easy to fix up in step 6.
3. Here I've laid clay over the whole thing at a thickness of 3/8". I've also laid in 3/8" strips that will act as keys.
4. Next, I put in a tube (paper towel or toilet paper tube) that will leave a space for pouring the silicone into.
5. Next, I make an jacket or mother mold. If you're going to bake the mold it should be Ultracal or fibreglass. If you're not going to bake it it can be plaster.
6. When whatever you've made the jacket out of is set you can remove it and clean it out. Remove the clay from the face (or sculpture, whatever) and make sure it's ready for final molding (touch up any damage that may have happened from the molding process so far).
7. Drill bleeders into the jacket to allow air to get out that may get trapped inside as you're pouring the mold.
8. Put the jacket back over the face and seal any joins where silicone could leak out. You can use clay, hot glue, etc.
9. Put a clean tube back into the space left and seal the edges where it touches the mold.
10. Pour your silicone into the tube. The silicone should be de-aired in a vacuum chamber but since most people don't have access to one just pour it slowly, in a thin stream, into the tube. This will help get some of the bubbles out of the mix. Let the silicone flow out of the bleeders until you're sure no air is left inside the mold. Then seal the holes. I find sticking a screw into them and just twisting it by hand about a 1/4 turn works well (as long as you have a small enough hole). You can also use a small ball of clay. 11. When the silicone is cured (usually overnight) open the mold up. When you've removed the face/sculpture your mold will look something like this.
12. You can now cast a part out of the mold. In this example I've made a dupe out of Ultracal and put an aluminium handle in it.
A side note here. The strip keys I used in this mold are not correctly shaped. they do not lock into the jacket at all. The sides of the keys are angled. They should be square. See the pic below for a visual description.