For my first covering jobs, I pretty much just went at it and sort of "made it happen", if that makes sense. I was in the same boat as you- very much concerned of destroying my own work, or at least wasting a lot of covering. If you screw it up real bad, you can usually reheat, remove, and try again with little consequence. It tends to be a lot scarier before you realize how easy it is
I suppose there are a few things, though...
- Set aside a good-sized chunk of time for your first attempt. Just trust me on this one.
- Experiment with scrap wood and pieces of covering to see exactly where your iron needs to be set to activate the adhesive and where shrinkage begins. It's easier to do this in advance than to try figuring it out as you start in on the model itself.
- When you cut pieces of covering to apply, make them a fair bit bigger than you need. It's better than cutting "just right" and having it be too small. A quick trim solves any excess. Also, don't fall into the trap of trying to use a single huge chunk to cover half the model in one go. Break it up a little... bite-size chunks.
- Don't start shrinking stuff until you've got most/all of the covering tacked in place, and then seal and shrink it "all at once". Some light structures can be severely warped (and thus damaged) by the force of the covering shrinking asymmetrically. It's not super-critical, but it's generally a good habit to fall into.
- If you're gonna be shrinking with an iron, just sort of "hover" it over the covering rather than actually touching it. This lets you see what the film is doing and keeps you from inadvertently putting pressure or force on it as it tries to shrink.
- Don't get too wrapped up making it all perfect and awesome. "Good enough" is usually good enough, and "just about perfect" is usually followed by "dammit". If you poke or burn a hole in your covering, use a round patch to fix it. No corners on patches; they'll lift up and peel off over time.
- Take a break now and then. Covering and wood are remarkably patient.
I'm sure others will chime in here... In the meantime, good luck