Here are some general points to read over and some videos that will help you as you begin to stipple.
Common sense - take your time, practice on some old A2 grips, magazine floor plates, old clinton era 10 round Glock 17 mags (unless you live in a state that requires you keep them), some of the practice material we sell here etc. This will give you familiarity with the tool/tips, and enable you to recognize some important things (outlined below). Then and only then should you move on to your firearm. If in doubt, take it to an experienced gunsmith.
Time, heat, and pressure. This is what controls the process.
Time: More or less time is required for some materials then others.
Heat: More heat is more suitable for some materials then others. I recommend a 25watt woodburner for most users and most jobs. They burn just under 950*F generally. A 40 watt unit would burn so much faster that it would require less time and pressure. Why not use a 40 watt then? Because it also makes it easier to mess things up. The tips are designed to increase productivity by covering more surface area rather then using and ultra hot tool.
Pressure: Pressure controls the flow of material as you melt it. Realize that is what you are doing, melting the gun's material, allowing it to take the shape of the tip you are using. Keep in mind different gun frames will 'melt' differently. Glocks are harder material then XD's slightly, the M&P's removable backstraps are super soft and require a gentle touch. Don't practice on a Pmag (one of the hardest things on the market) and then jump into your Glock and expect the same results while utilizing the same time/pressure.
Some important notes:
Most colored gun frames will not retain the factory color, as the waffle tips will leave a small amount of material on the tip, resulting in your frame having a black tint to it in spots. You can mitigate this by keeping the tip VERY clean with a brass/copper wire brush.