I went to thrifting and found some sheets - in fact, I got a california king set and 2 flat full sheets for less than $4. :)
Then I went to sewing.
First, I sewed the two full sheets together along their "long" side.
Then, I laid the king sheet out flat on the floor and laid the sewn together full sheets on top of it. They didn't quite match up, the full sheets were a little longer, but I figured that's ok because the fort will just have more space, so I kind of "pleated" the top fabric...I just folded it over every once in a while to pick up the extra slack so that when I got to the corner they matched up. If you are wanting your fort to be beautiful or your sheets have patterns, you might want to make sure that you have the "inside"sides facing.
Now....just sew 'er up. You'll want to leave a space of about 3-4' un-sewn at one end to allow access (by people) and to allow room to fit in your fan.
Spread the fort out, put in the fan, and BAM Instant FUN!!
We had a little picnic, did some reading, even had a small pillow "fight". Lots of giggles and all kinds of fun memories!
**Of course, any time kids are involved with something like a fan - parental guidance is necessary to protect their little curious fingers! But who wouldn't want to hang out in a fort all day? :)