Fort building is an art, and it will be no easy task to make an Instructable on this subject, mostly because your imagination and ability to improvise will be your biggest tools. But nevertheless, I will start with some basic forts and then go on to show you some improvised ones. There are two basic parts to a good fort: sturdy support, and covering.
You may need:
Chairs, desks, bookshelves, etc.
You may need additional supplies, so be prepared to get up and walk (I know, I'm so cruel.)
Step 1: The Two Chair Fort (Step 1)
Here's a basic one: two chairs and some blankets. Step one: arrange two chairs back to back 3-4 feet apart. Your blanket should go down the back and just touch the seat.