Depending on how complicated you want to make it for yourself, you can build supports, prepare half of the mold at a time, etc etc etc.
I personally am lazy and don't feel like making a federal case out of the process, so when I do a two-piece mold it's a fairly simple process. If you want to use one of the materials that you pour, or something that's really large, this might not be the best way to do it, But for small items this will work just fine.
For this mold I decided to use a gumpaste poinsettia center that I'd made before, since I've been thinking of making some for a display cake, and having a mold to use for the centers would speed the process up.
This was a bunch of wired stamen-type shapes that were all gathered together, so it was already on a wire. I could use that for the guide as to which side should be up.
I mixed the silicone according to the instructions, and then rolled it into a smooth ball.
Flatten the ball out and wrap it around the piece that you're using.
If there are a lot of crevices, like the center that I was using has, try to get the silicone around as much of it as you can, but realize that the deeper the cavity ends up, the more difficult it will be to extract the final piece from the mold. You don't want the silicone to wrap down into any really deep parts of the object, in other words.
Try to keep the top at the top of the silicone ball as you wrap it. If you can't attach something to the item and it isn't perfectly round, this can be slightly problematic, so just do your best.
When you're ready, take an exacto knife and carefully cut the mold open. Try to keep it as symmetrical as you can, and start with a shallow cut all the way across the silicone ball.
Use the knife to continue cutting the interior part of the silicone while you're pulling the two parts away from each other.When the cuts are deep enough you'll be able to completely pull the sides apart.
Pull the item that was molded out of the inside of the silicone. At this point there could be a lot of deep crevices that would be hard to remove any modelling material from. Examine the mold to see where you'll need to remove part of the silicone.
This mold had a couple of really deep sections, so I cut two of the deeper flaps off. You want to create enough of an open space that you'll be able to pull the material out of the mold, but not so much that it will ruin the shape of the finished piece. Start with removing a tiny piece, then remove more if you need to.
Press some gumpaste or candy clay into the mold to test out the impressions that you get from each side. If it looks good, fill one side with gumpaste then fill the other side.
If the mold is rounder and doesn't have as many protruding shapes, you can put a ball the size of the finished piece into one half, then press the other half of the mold onto that. Since this one has a lot of protruding pieces, I pressed the gumpaste into the two individual cavities to make sure they were both full, then pressed the two sides of the mold together.