Just follow the instructions on the can...it's pretty basic.
General rule of thumb is ten parts Bondo mixed with 1 part hardener (golfball size amount of filler and pea-sized amount of hardener, give or take. The exact ratio isn't that critical, just keep it in the ballpark.) The worst that can happen is you'll have the stuff set too slowly (not enough hardener) or it'll set up too fast (too much hardener). After you mix a few batches you'll get a feel for it. It's really pretty simple. Just mix small batches at a time. If you mix a big batch it'll start to set up before you can apply it all.
There's not really a whole lot more to it. Mix it up, apply, wait till it cures and sand smooth.
For smaller areas or to fill scratches or smaller imperfections you might try their 2-part glazing putty. You mix it up the same way as the canned stuff, but it's more convenient to use, because the filler comes in a squeeze tube, like the hardener.