Step 1: Starting
carefully undo the sides of the top to open it out, like so.
Cut off the white parts, so it looks like this.
Step 2: White Triangle
Cut a triangle into the top portion in the middle, so it looks like this:
Step 3: Folding
Fold the white top part around to look like a collar.
Step 4: Final Step
Reopen the previously bent in sides from the back, viola! You are done!