The basic premise behind making a mold of an object is encasing it in a somewhat viscose material within an airtight border, and letting that material harden around it to make a negative. The negative of the object is what becomes the mold.
The first step to making a mold is to make an airtight border that is larger and taller than the object you want to copy that allows the casting material to fully cover it. Next, use a releasing agent, such as Vaseline, on the object to ensure it is easy to remove from the cured, or hardened, casting, and then pour the mixed casting material over the object. If the object has a lot of detail that you would like to copy, paint a thin coat of the casting material on the object before pouring the rest. Then, allow the casting material to set, and then remove the original item.
If the object is a complicated shape, like an action figure, create a two-piece mold to make it easier to remove the original object, while maintaining the integrity of the mold. You start similarly but you put clay into the border first and press your object halfway into it. Again, a releasing agent is recommended. Apply the casting agent like you would before. Once it has cured, remove the clay and do the same process to the other side. To make sure both halves line up properly, you can cut sections out of the clay to make a key for the other half to fit to.
Once you have the negative mold you can fill it with a hardening material to make a copy.