It's really rather simple, once you have the speedometer out of the dash. Usually there is a stud lock holding the gauge in the dash. Examine the back of the dash and rotate the gauge so the stud lines up with the hole and remove.
It is held in place by three small studs that are mounted in the case of the speedometer. Turn it 15 degrees in a clockwise direction as seen from the front of the gauge and the studs will line up with three notches in the steel of the dash.
Once you have the studs lined up, it's just a matter of working the gauge in a side to side motion to get it out. This assumes of course that you've disconnected the cable and lamp already. Once you have it off, the face is held on by tabs around the edge. Don't pry them, that can lead to metal fatigue and once they break off you'll have to order a new bezel. Just turn the entire bezel so that the tabs line up with the notches in the gauge.
Tip from Mike Sowder: There always comes a time, during restoration, when you are faced with the challenge of removing those damned chrome rings from the instrument panel. A lot of trial and error finally led me to use yellow oven cleaning gloves. It is amazing how much grip you can get with them. Try them, they work.
Your original gasket will be completely useless 99% of the time. They generally turn into a hard carbon-like substance. You can get new ones from Nisonger. You'll have to scrape the old gasket out with a small screwdriver. Once you have your new gaskets and lens, you just put it back together in reverse order. It will be tight due to the new gasket, but it will go back together. There is a small tab at the 11 O'clock position under the bezel on the gauge that holds the lens from rotating. If your new lens doesn't have the notch that the tab fits into you can just bend it up out of the way. The lens shouldn't rotate without it. Good Luck!