Here's how I do it. First, remember that these molds do not last! The more times you pour the weaker they get, plus if they are not made just right pulling the finished product out of the mold will cause it to break. For that reason you have to accept that they won't be perfect by any means. If you can live with that:

The Plaster mold should be about an inch thicker all around than the thing you are making. So you needa small box that will accomodate that. For round weights I like to make a replica out of styrofoam or buy a round shaped object at a craft store. The box should be generously coated with vasoline (inside of the box). This will allow the mold to come out after you have made it. The sinker mold also needs to be covered with vasoline. Mix up enough PP to fill the box 1/2 full. When the PP is smooth, pour it into the box. As it begins to set take your mold and press it 1/2 of the way into the PP. You need to do this at the right moment - if you wait too late the PP will be set and the mold will not sink into it easily. Be sure to have attached a eyelet into the sinker mold and have the eyelet go just into the mold. Add 4 small nails, one at each corner of the box (insert into the PP). Let the PP harden. After it is hard, lather the top half of the mold, as well as the nails and the inside of the box liberally with vasoline. Now mix up more PP and fill the box to the top. Your sinker mold has been covered up. Let it dry. After it is dry, if you used enough vasoline, the whole mold should slide out of the box. Then, take a screwdrived and GENTALLY separate the two halves of the PP mold. Remove the sinker mold. The nails will come out of the top half of the mold but stay in the bottom half. Clean all the vasoline off. Take a chisel and trim out a lead pouring hole. Bake the mold to remove moisture. You now have a mold that will work about 10 times or so, gradually it will start to chip and crack and fall apart.

After reading the above, the big question is why you would bother to make a mold that will fall apart after 10 uses? It's a lot of work! Personally, I wouldn't bother for a round sinker mold. I would buy the mold. BUT, I make molds for jigs like Point Wilson darts, etc. I make my own balsa wood shapes and then make the molds. Now it becomes worth the work because for the cost of materials you are making a $3-4 jig. And they work great! I also make a "half jig" that is very simple and I can make a hundred before the mold falls apart. The jig is used for curly worms for salt water fishing and works quite well. If I get a chance I'll post a picture.

It's fun making your own gear when possible. I try to wiegh the amount of work (time) vrs just buying the gear. I make my own halibut spreaders and leaders (quite easy). I once made flashers (much harder) but it wasn't worth the work. I just wanted to see if I could do it.

Good luck if you decide to do this. I would recommend going to the library and looking for a book on making your own tackel. I found one several years ago that gae me a lot of great ideas, sorry I can't remember the name of it.