There are three simple steps you should basically fllow to assemble an do-it-yourself kit. There explained below, please refer to the image (also shown below).
Completely insert the spring #1 into the inner cavity of the knife.
Completely insert the pivot #2 into the hole. Now close the blade and try to open it automatically.
If the knife works correctly, insert the rivet #3 into the pivot #2 hole and hit it with a little hammer.