I use a board like a 1x3 or 1x4 to level the structo-lite and the first coat of durabond. If you fill the area to over flowing with durabond and then knock off the excess, resting the ends of the board on the surrounding wall as a guide, it will give you a good starting point for applying the finish skim coats. The first coat of durabond will be rough when it sets. You can scrape off any bubbles or ridges in the surface after it sets if you do it within the first hour or two. After that it gets much harder to remove. Use a drywall joint knife to scrape or you can sand the set mud with 50 or 80 grit sandpaper. Smooth the edges of the mud at the same time.
To finish I use a 6" drywall joint knife to apply several thin skim coats of durabond. You can even use ready-mixed joint compound for the last couple of skim coats to make final sanding easier. We have a website with illustrated instructions for repairing all types of wall damage. I can't post it here but I'll email the url to you if you want. My email firstname.lastname@example.org