We have been in the concrete ornament business for over thirty years; and because molds are very expensive, we chose to make a few of our own. Copyright laws are not just for the printed page, almost anything anyone mass produces; especially yard ornaments have a copyright. If you find the cutest plastic or fiberglass gnome at Fred Meyer, do not take it home and make a mold of it unless you are going to add it to another item or change the entire figure into something completely different. For example; take two gnomes of the same size (in different positions) and place them together under a mushroom. That would be a very complicated mold because of the seams and undercuts required to get it all out in one piece, so I will give you a simpler task.
If you have an item that has no undercuts and is wider at the top than at the bottom, you can make a sleeve mold with no seam, but larger items will have a seam. The place that sells the liquid rubber will also sell clear sheeting for making seams; these are either completely flat or with various sizes of bubbles in them. These bubbles help keep the seam straight when you are actually ready to put together and pour the mold. First find an inconspicuous place on the figure to place the seam and then cut the plastic the length and shape of the figure. If your cutting is not exact, you can use a bit of clay to fill in the "holes". The seam piece needs to be at least two inches wide and remember also that if your figure has ears (like on a deer) or say arms on the hips with "holes" in the center, you will also have to put your seam material there too.
Once your seams are on and your item is on a flat surface, begin painting a thin coat of the liquid rubber on the figure, both sides of the seam, and at least a two-inch wide circle at the bottom of the figure, thus attaching the figure to the board. Do not worry about it being attached to the board; it will come off after you are done with your project. Once the rubber is thick enough (anywhere from 1/8 inch to a quarter inch thick), let the item sit for several days to be sure it is completely dry and cured.
Next you will need another piece of seaming material or maybe even two (this is where the bubbled seaming material is really handy). Place this on your completed project the same way as you did when you needed the seam for the rubber. Whether this figurine has undercuts or not, it is likely three dimensional and the plaster on the outside has to be taken off without breaking the poured figure (or the plaster). If the figure is not too complicated, you should be able to place the one seam opposite the original seam giving you two half molds; if not, you may need the plaster to come apart in three sections. A thing to consider during this time is how you are going to keep the mold together. If the mold is small enough, you can use tie-wire that you wrap around the mold about a quarter from the bottom and a quarter from the top and tighten. If it is a large mold, you will want to include metal angle pieces with screw holes. Three on each side of the seam so that the screw holes match up enabling you to hold the mold together with bolts and wing nuts.
Mix up your plaster in about a gallon bucket; it should be about as thick as oatmeal before you add the milk. Work on one side of the mold between seams building that section up. If it is slipping down the sides, it is a little too thin and you will need to add a bit more plaster. You need to work quickly as plaster does set up within 15 minutes. You will probably have to do several layers letting each layer dry. It is during this time that you will insert your metal angles in between the layers of plaster. If your figurine is large, you can add reinforcement to the plaster with webbing or other fill agents that are available from the rubber or plaster manufacturers. Once you have one side done and the plaster is dry enough to not break when you move the entire thing, take the dividing seam off and rub some tree wax onto the plaster seam. Then begin adding plaster to the next section the in same way that you did the first, follow the same procedure if you have a third seam.
Once all sides are done, cut another plastic seam and place it on the top of the plaster where the plaster will divide. You need to add more plaster to the top part of your figure on either side of the seam. Place some wax paper on top of this mixture while it is still wet and press a piece of plywood on the top (please use a level) so that it is flat. The reason for this is that the mold is poured upside down and will need to have a level stand. If you are using the metal angles, you need to check to see if the screws can go through the holes without hitting plaster and might need to chisel off a little in order to make the screws slide easily.
Now that you have this gigantic mass of plaster in front of you it is time to break it apart. I usually do that by slipping a couple of screwdrivers into the seam and gently wiggle them to pop the mold apart. After the plaster is off, let it air dry for at least three days, but preferably a week. Damp plaster will mold and deteriorate the rubber. Now it is time to take the rubber off the figure, gently pull up from the bottom and begin separating the seams. More than likely you will have some rubber that leaked through the seam, but if you are careful you can slice it so that it comes apart easily. Also along the bottom there may be some extra overlapping rubber that you need to trim. Once the rubber is off, you should have a continuous wrap-around piece of rubber that is the exact replica of the figurine. The rubber should fit nicely in the plaster when it comes time to put it together. Bolt (or tie) the plaster together making sure that the rubber seams are good and tight and that the rubber is not being pinched where the plaster fits together on the other side. This is easily done by placing the mold in the upside-down position and running your hand in to be sure there are no pinches and that the rubber fits nicely.
Mix your cement as thick cake mix, pour and vibrate the mold to get out the air bubbles. This can be done with a purchased vibrator, or by shaking the mold, or prodding with a stick or piece of steel rebar. Check back several times as the concrete will settle and water will rise to the top, so you will need to add a bit more mix. Be sure that the top (which is actually the bottom) is level, let it sit for a couple of days - or even a day if it is hot out. Take the plaster off, peel the rubber and you have a perfect replica of the original. You just need to file any rough seams and then S-L-O-W-L-Y cure the statue by keeping it wet for a week in order to make it strong. Then be sure it is completely dry for sealing and painting if you so desire.