Papercrafts come in different forms.
Here are the most common used file types and how to open them.
.pdf files are opened with Adobe Reader.
.pdo files are opened with pepakura Viewer/pepakura designer. (windows only)
.zip and .rar files are opened with WinRAR.
Once you opened and printed these files, you will have to cut the papercraft.
You can choose whatever works best for you. Cutting knife, scalpel, scissors,...