The short and simple method is this:
If you are changing weights of the oil go to step 1, if not, skip to step 2.
1. Empty shocks as best as possible and clean with denatured alcohol. You want to get out any of the oil possible.
2. With an EMPTY shock, remove the cap and extend the shaft of the shock all the way to its ready position.
3. Fill with desired weight oil until you are about 3 mm away from the top edge of the shock body.
4. Plunge the shock shaft up and down until most of the air rises to the top. You do not want air in the shocks if you can help it.
5. Topping off the shocks is simple. Compress the shock until the plate inside the shock body is ALMOST to the top of the shock body. Then, top off with oil.
6. With the plate still at the top, put the cap back on slowly. Some oil will run out. This is good. This is bleeding the shock.
All done.
One thing you do NOT want to see a shock do is extend by itself. If it does, you have too much oil in it. This is what usually will result in a blown shock cap.
If you have a shock jig it makes like easier to fill and you can let them sit and ensure most of the air comes out as some can take a long time to rise. You can easily make your own jig with either thick foam from something like a monitor box or drill out some wood etc...
If you need clarification on anything, let me know. Maybe I will do a 'How To' and go a bit more in depth and put it on the site.
3.