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.
Step 2: The Two Chair Fort (Step 2)
Get a nice sized blanket. Find one of the two shorter ends.
Step 3: The Two Chair Fort (Step 3)
Drape the short end of the blanket over one of the chairs. If it doesn't reach the ground, put blankets on either side, like I did. If you like the dark, drape small blanket over the ends of each chair. As a finishing touch, especially if the ground's hard and cold, you can put a blanket on the floor. Enjoy!
Here's a short anecdote. When I was making this fort, my dog went inside. The blanket dragged on him and one of the chairs fell on him. He freaked out and jumped back, but he was still trapped by one of the blankets. I set the chair back upright, and the other one fell on him. He escaped from the blanket and RAN!
Step 4: The Desk Fort (Step 1)
Start with a desk. (Duh.) Put chairs on either side with the backs facing inward.
Step 5: The Disk Fort (Part 2)
Now put the short side of the blanket onto the desk and anchor it down using books (Phone books, with a small face but lots of pages, are preferable.) Put the rest of the blanket on the chairs and move them back until the blanket is tight.