You will need:
(1) Your digital camera - I used a Canon Ixus 65
(2) An old or broken SLR zoom lens - I picked up a Sigma 28-200mm from the broken lenses box of a camera shop for 5 quid, I think the autofocus didn't work.
(3) some really tiny screwdrivers, these SLR lenses are held together with tiny screws
(4) Luck - plenty of it
(5) Black electrical tape, various tubes, superglue, elastic bands, anything else you can think of to make a mount for the lens.
Step 1: Take the Lens Apart
You are after the lens element that sits just behind the front lens element. Keep all the other bits for other projects.
Hold it up an look through it, does it appear to show you a nice wide image?
Step 3: Test the Lens Out
You may need to put your camera in "macro" mode (the mode you use to shoot pictures of close up objects) to get it to focus correctly. Try playing with the zoom options on your camera as well, you may have to play about to find the sweet spot.
You may also need to vary the distance from the front of your camera lens to the fisheye lens.
You are after a nice crisp image but be aware that there may be some blurring and distortion near the edges.
Step 4: Add a Tube
I started by adding a tube that holds it just the right distance away so that it all focuses correctly and adds a bit more stability.
I used the plastic inside tube from a roll of sticking plaster, this just happened to be the right diameter to attach to the fisheye lens and the right length to produce a sharp image.
You will have to raid your spares box and experiment a bit to find something that work for you. Plastic tube is good as its soft and easy to cut and adjust.
Wrap some black electrical tape around whatever you use to keep any stray light or reflections spoiling the image.
Test it out, see if it works for you, you will still have to hold it but it will be much steadier.
Step 5: Make a Mount
I tried a few different ways including holding it on the front with elastic bands.
In the end I had a metal mount made by a friend as they have a workshop and various metal bending tools.This mount allows me to mount the camera and lens to a tripod or my bike so I don't have to hold it all in place. It makes it very easy to make fisheye films.
You may be able to make one from card wrapped in duct tape, a bit of road sign that you acquired or from the side of an ice cream tub or box., it should be fairly rigid.
The mount is secured to the camera with a quick release plate from my tripod. The bolt passes through a hole in the bottom of the metal mount and screws into the tripod mount of my camera holding it all together tightly.
Some cameras (like mine) don't have the tripod mount directly in line with the centre axis of the lens, you will have to allow for this offset when designing your mount. "Measure twice, cut once" is the best advice I can offer here :)
Have a look at my set on flickr for some more fisheye photos.
You can use FisheyeWarp to do clever things to your images, its free!
Have a look at this Flickr Group for DIY lenses
Go out with your new lens, shoot everything again in super wide vision.