Party top-hat made of playing cards

Step 6 Staple top cards makng the ventilation hole

You can actually do this later but it'll make structure stronger and will help you with handling whole thing on later stages so I suggest you do it now.

I used special (tiny) stapler to do this but with some practice you can do this with a large one too. Just staple through cards - the more staples goes through cards the stronger hat gets.