Step 1: Step One: Fill Your Torch
Step 2: Step 2: Polish
Hold the torch at a low angle across the material. You want to heat the surface, not the whole thickness of the plastic. Move the torch continuously as if you were painting: you want the plastic to reach the temperature just before flame up, but not actually catch on fire. As soon as you see the surface turn shiny (or just before), you'll want to move the torch away: lingering too long will cause it to flame. That'll ruin your surface. If a small flame does occur, you can blow it out right away. Don't push the boundaries though: this is plastic, on fire, so if things get too hot, put safety before your project.
If a small flame does occur and you blow it out, you'll notice a the surface will look roughed up and distressed. Let it cool off, completely, and hit it again with the torch, being extra careful to move on at the right time. If at all possible, you should never torch the same area twice. Though acrylic is somewhat forgiving, you don't really get do-overs!
Have fun, and be safe!