With Halloween coming up fast most of the things I have been making are of a spooky nature, but really you can go any direction with this.
I'm sure we've all seen a video or two about making moulds from silicone and dish soap, but the real trick is finding that perfect object to make a mould of. I got sick of looking and finally decided that if I can make the mould I can make the object too; and really the sky is the limit if you have some patience and creativity.

Step 1: Materials

For the object you want to make copies of all you need is some pottery clay; if you are making something big you may want to have some filler(I used aluminum foil). It also helps to have a few simple tools and a good picture of what you are making, but you can do without if you don't have any.
For the silicone mould you will need:
Silicone (duh)
Dish soap
Large bowl
You can get the silicone for a few bucks at Walmart or any hardware store. I used GE brand type 1 silicone because it's incidental food safe (I'll talk more about that in a bit).

Step 2: Make the Object

This part is by far the most time consuming and difficult, but if you can't seem to find what you're looking for already made then it's a great way to make exactly what you want. And of course you can always just use an object you already have if you don't like the idea of making it yourself, I'm just particular and I enjoy a good challenge.
I had several things I wanted to make, some were pretty simple and some complex. Here are some pictures of making the brain and skull. I used a ball of tinfoil in the center so they would be lighter and not use so much clay. The trick is to be patient and maintain a good moisture level, not too wet or too dry, so that the details are easily formed and come out right. It also really pays off to have a good picture for reference. When it's how you like it let it dry completely(a day or two) before going to the next step. One thing that's nice about this is that you don't have to fire the clay- just dry it out, so you don't need a kiln.
I realize you can get moulds of brains and skulls, but this is way cheaper and none of the ones I could find were quite what I was looking for (especially the skull, I could not find one without the jaw).
Pottery clay releases easily from the mould, does a good job of preserving details, and it's easy to work with. Another benefit is that when you are done you can soak it down and reuse the clay for other projects. If you don't have any you can try something else, but I don't know how it will turn out.

Step 3: Make the Mould

I don't know who discovered this method of making moulds, but it's awesome. And if my explination doesn't make sense there are several videos online. But it's pretty simple so I don't think you'll need them.
Fill your large bowl with water and add about a tablespoon of dish soap (like dawn, not dishwasher soap) and mix it together. Cut the end off of the silicone tube and put it all in the bowl (you're going to want to use a caulking gun for this). Now get your hands a little soapy and start squeezing the silicone into a ball, make sure you keep it submerged as much as you can while doing this or it will start to stick to you.
When it's all squished together and holds that shape (for the most part) then it's ready to make your mould.
Put a little bit of soap water on a cookie sheet or table, or whatever surface you will be working on so the silicone doesn't stick to it. Dip your objects in the bowl of soap water and press them into the flattened silicone, being careful not to press so far down that they go through the bottom. For larger objects you will need to squish the silicone around it (rather than just pressing it into it). As you do this be sure to press it into the cracks so that it picks up all the details and don't cover the whole thing or you will have a rough time getting it out.

Note: Silicone has a strong vinegar smell that can sometimes make people feel a little sick, so it's best to do this in a well ventilated area.

Step 4: Clean and Use the Mould

Let it sit for a few hours (between 2 and 5). When it's good and cured go ahead and get the object out; it may be a little difficult at first, just gently peel the edges back all the way around and work your way in patiently.
Clean the mould out with warm soapy water, making sure you get in the cracks.
The first time I use a mould I like to just fill it with water and freeze it so I can see how it will look without wasting plaster or jello etc. This also helps you get a feel for how to get stuff out without the risk of breaking a project you're working on- no big deal if the ice breaks.
If you are using the mould for plaster or other non food uses you can use it right away. If you are using it for food it will still need to cure for a few days(you'll know it's cured when the vinegar smell goes away).

Important Note: regular silicone is NOT rated for food use; this us why I mentioned type 1 incidental food safe silicone(IFS). But even this type is not like the cupcake moulds you get in the store, it is only safe up to 200°f. You can not bake things in these moulds or microwave them.
I did a lot of reading on this and the only silicone I could find that is completely food safe is a 2 part mix and pour type. If you attempt to use IFS silicone above 200°f it will begin to leach chemicals and is no longer food safe. That being said, you can still use it for gelatin and chocolate as long as you watch your temperatures.
Also, it's a good idea to keep moulds for food separate from moulds that have had non food items made in them as it may contaminate them.

Step 5: A Few Tips

By trial and error I have learned a few things about making a functional mould.
First, make sure the object you are casting does not have any holes that go straight through, or that they are not covered on both sides, otherwise you will have to cut the mould or break the object to get it out. This also goes for any shapes with deep crevices; if it's too deep you will have a very hard time getting it out without ruining something.
Something else that's helpful; when casting something 3D, you are still going to have a flat side where the opening is. Be sure the opening is not less than half the size of the object at its widest, the silicone can stretch, but only so much.
Also, be sure you knead the silicone really well and get all the bubbles out or you will end up with a rough surface.
And lastly, be sure that the mould is plenty thick. If it has thin spots they will tend to bulge when filled and your project will come out deformed.
Thanks for reading; there are so many possibilities so get creative and enjoy!