Cutting Technique
The first step in making a papercraft model is to cut out your pieces. You can either cut all your pieces at once, or cut a single module, score, fold and glue it, and then move onto the next module. I prefer to go one module at a time, so that I actually see things being built. It motivates me the best. Some people use a ruler to cut straight lines. I've found that while the lines will be really straight with a ruler, the accuracy is often an issue since it's difficult to line up a ruler exactly with your cutting line. Therefore, I tend to avoid rulers. Here are some tips on cutting out your pieces accurately.
1. Don't begin by cutting the detail. Roughly cut out each part first, then trim away the edges by following the lines.
2. Rotate the paper you're trying to cut, don't try to contort/rotate your hand to get the right angle. Cut towards yourself.
3. Keep the blade not inside or outside the lines, but exactly on the lines.
4. When making cuts of outgoing corners, cut a little bit further than you need to, so that your cuts cross each other and pieces pop out easily.
5. Store all pieces in a bowl or envelope so they don't get lost.
6. Replace blades often to make sure your cuts are clean and your paper doesn't end up tearing.
Folding Technique
The first step before folding is to score all the lines you're going to fold on. Use your scoring tool to trace along all fold lines. Try to keep your scorer moving in a straight line and apply even pressure down on the paper to make a crease. This will help you fold the paper precisely.
When folding, be gentle and let the crease you made dictate how the fold will proceed. If you're using Pepakura Viewer, look at how the piece is supposed to look and fold each crease the appropriate direction. Try not to fold pieces all the way back (180 degrees) if they don't need to be at that angle. Fold to the angle the piece will need to be at for gluing.
Gluing Technique
I know gluing things together doesn't sound hard, and it isn't, but there are some tips and tricks that make gluing a little easier.
1. When starting, place a glob of glue on a sticky note to use. This will make it easier to not overuse glue and get things soggy.
2. Place glue, not on the tab you want to glue, but on the area the tab will contact.
3. Hold the two pieces together for at least 30 seconds to let the glue set before letting go.
4. Glue one tab at a time. Don't try to put glue on all the tabs at once.
5. You can use your finger for larger areas, but I like using a toothpick for the more intricate tabs and glue locations.
6. When gluing box shapes, place glue on the edges of the box and then push in the top with the tabs.
7. When gluing together two parts, place glue on the part which will less likely show excess glue.