Use a craft knife to cut an opening in a part. Place the sheet on a flat surface and follow the outline with the blade. A cutting mat protects your table and helps preserve the blade. Turn the paper as you cut rather than twisting the knife in your hand; that is easier and reduces the risk of breaking the blade.
For an opening too small to cut normally, pierce the paper instead. The waste piece should fall out by itself. Do not pull it free: an incompletely cut edge may tear. Pierce or cut the stubborn point once more.
Small manicure scissors and a craft knife are both useful for cutting tiny pieces.
For angular shapes, start with straight scissors and finish hard-to-reach corners with a knife. For rounded shapes, curved scissors can make the first cut easier.
Colour exposed cut edges with a felt-tip pen, pencil or marker close to the colour of the adjacent surface. This greatly improves the finished model's appearance.
If a small part needs to be rigid, carefully impregnate it with superglue. Once dry, it will be stiff, almost like plastic.
Tweezers are very helpful for pieces too small to pick up easily with your fingers.
Which tools do you use for small parts and openings?