Square Puzzle

Some of you may know it from before. These puzzles were occasionally given as a giveaway. Since after years i keep forgetting the solution and desperate about it and now i thought to have to share it with you :D. The solution is not that simple, but you can do it. If you can't figure it out, just write me i can give you a tip. You don't need any other tools to assemble. There are 2 versions of the lid of the box in case it should be a bit difficult to get into the box. And again to the task: The 5 parts must be placed so that they form a square.