step one: put two chairs a few metres apart in front of a couch
step two: put a sheet over the chairs and the back of the couch. if you have a broom handle, stick that in the middle like a tent post and it will keep your fort from sagging, (not pictured here).
step three: get in the fort and bring other stuff in there with you. i suggest someone you like, some pillows, a bottle of wine, a little mood lighting and a phone in case you get stuck in there and can't go into work the next day.