I know there are a lot of resources over the net of how to build a steadicam, but i thought i would share my design and influences in case it helps anybody.
I found the below video useful for explaining the principles of a steadicam, which really helped me understand how and why a steadicam works, and what needs to be taken into account when building my own.
To summarize, you need to counteract the camera's movements in 3 ways - Pan (Side to side) Tilt (Up and down) and Cant (Twist). If these 3 movements can be isolated, the whole camera's movements can be.
The ways to do this are by creating a device that allows movement in those 3 directions, a Gimbal. A gimbal is a device that allows movement in multiple directions through one point of rotation. This can be a ball-and-socket joint, or the kind of thing you see here - http://www.yb2normal.com/DIYsteadicam.html
When you have this Gimbal, a camera attached on to it, and some sort of weight to keep the camera held down, you have a basic steadicam. The idea is that the gimbal allows the shake that would be present in the camera footage to be compensated for, reducing much or what you see.
For my design I used:
A Tripod head (I had this already and it means easier mounting and greater control, but there are many other ways that you can attach your camera)
1 metre long 10mm threaded screw pole (This thread fits the thread in the tripod head)
1 metre 8mm threaded screw pole
8mm lock nuts (I think i used about 12)
10mm lock nuts (I used 4)
washers (quite a few)
a spinny bearing thing
I chose this bearing thingymabob (sorry i don't know the technical term for it) as it had holes which i could screw through in order to attatch it to some wood. I cut a square out just bigger than it out of MDF, then that screwed on nicely with some short screws. I marked out the center of the MDF by drawing lines across the corners, to drill a hole through, big enough for the 10mm threaded pole to fit through snugly, but freely. This MDF is what the pole will be attatched to, using lock nuts. Then, on the other side of the bearing I screwed on another square of wood, this time with a square cut out of the center (this was done using a jigsaw through drill holes). This wood needs to be relatively thick, so you can drill a hole in 2 opposite edges, as close to the center as possible (See the picture).
The next step is to measure and cut out two larger squares. As you can see, the largest square also has a handle coming out from it. To get the sizes for the larger squares, i simply cut a square about 4cm by 4cm larger than the previous, then cutting the center out about 2cm from the edge. This meant there should be enough room to fit lock nuts and washers in the gap. Then holes need to be drilled in each of these pieces of wood, on each edge for the middle piece, and on two opposite edges on the outer.
Now comes the trickiest bit. I needed to cut lengths of the 8mm threaded pole using a hacksaw, and lock nut them inside the drill holes in the wood. The great thing about lock nuts is that they are unlikely to come loose when screwed into place. The difficulty with them is that it is also very hard to screw them on in the first place. I used two wrenches, one to hold the pole still and another to turn the nut. You must put washers on the pole too, to be at the edges of the wood. This way any pressure the nuts apply to the wood is spread out, so it won't damage the wood as much.This took me a good couple of hours to do, but it was worthwhile as the nuts would just come off if i hadn't used lock nuts.
Then I threaded the 10mm pole through the hole in the center of the Gimbal, the hole drilled through the MDF. When this was lock-nutted in place, it meant that half the bearing was attached to the pole, and the other half spun freely, meaning the handle can rotate without affecting the camera.
Finally, a length of wood needs to be attached, with equal weight either side. I chose to cut two 2Ltr bottles and use them as containers for the weight, using stones, or sand. This means I can adjust the weight if I need to, and also it won't be as heavy to carry around when I'm not using it. I originally tried using a hollow plastic ball (Designed for floating for use in plumbing) filled with water as the weight, but the weight needs to be spread out as it is to balance it out, and keep it from making a pendulous swinging motion.
Here is a video of some test shots i made with the steadicam