How do you replace the timing belt on a 92 Mazda MX3?

If it's the V6 (can't answer for the 4), it's not easy, a lot of the engine accessory area has to be disassembled for this and valve cover removed (requiring new valve gaskets which are probably leaking anyhow), etc. Anything regarding timing belt (or water pump), and front wheel bearings, I consider "non-user serviceable", it's something for Mazda or a reputable Mazda certified repair department to handle. For the timing belt, simply go to Mazda, tell them you need your 60,000 mile (or equivalent multiple) service performed, get your wallet out to the tune of $840 or so USA dollars, and have alternate transport for a couple of days. This will get a lot of stuff cleaned and checked up, if you buy the parts separately, including replacement gaskets (valve cover (front required, both should be done though since you are in there), timing cover gasket, etc, you'll end up spending more for the parts than Mazda does for the overhaul, trust me on that.