The process of building a fort made of chairs and blankets is a long established pastime of formative years, which can be revisited time and time again, especially on cold rainy weekends. Normally the fort is built in the living room, den or dining room area of where you live, in order to defend the room from the boredom, ennui, and stuffheadedness of people you live with.
The basic FMoCaB is built with four chairs, blankets (or sheets), heavy items and (optionally) a broom. Align the four chairs in the centre of the room with the seat facing outwards. Throw blanket on top. Move blanket around. Pull chairs further out until edge of blanket hangs a bit over the seat to the floor. Watch blanket fall off. Mutter about it, then repeat above, but put something heavy on the seat of one chair to keep blanket in place. (weighty materials include: Dictionaries, encyclopaedias, Harry Potter books, bowling balls, the briefcase of the breadwinner in the house, a soporific pet or sibling, someone else's laptop, barbells, boots, iron, or anything else within easy reach.)
Usually, one blanket is not enough to make a suitable fort covering, so angling one blanket, weighing half a section on one chair, and two thirds on another chair (magically, the maths of this work out just fine) and then repeating with another blanket or two for the rest of the chairs will ensure a well-covered FMoCaB. You might want to use a broom inside to poke the blankets higher; inevitably this will pull the blanket up, sliding the weights on the chair off and thus collapse your fort, but after several attempts you will finally find the right balance.
Ruminations on sharing the task of building a fort:
It is not recommended to share fort building with someone else, unless that someone else is someone you have shared in the building of something successfully before. Learning the physics of fort-building's checks and balances is a tricky thing, and interference from know-it-alls, busybodies, and lazy armchair foremen offsets the process in an unfortunate manner upsetting to all. It is recommended that once you have built the fort, to invite others to join you inside, at least after a while. You may have intended to build the fort so you could be alone and be in your fort alone and that's that, but inevitably you will come to the realisation that the life of a solitary fort dweller is daft and rather dull, punctuated by long naps.
The main purpose of forts is protection from hostile forces, a place of harbour for those dwelling outside the fort to retreat to when either under attack, or addled with the notion that they may be attacked. Thus, when others arrive under the guise of judging the workmanship of your fort or to draw you out into games of frivolity and capering, invite them in to discover for themselves the comfort and safety of your FMoCaB, for they are actually calling out for your benevolence and goodwill and crave to share your mature foresight in creating a place of shelter. If you live almost alone, don't worry: eventually a maternal presence will ask for admittance and share your space with a cuddle and most likely sleeping, to acknowledge the wisdom of building a fort to escape from the onslaught of life's demands on all of us.
Variations on building a FMoCaB:
Replace one or two chairs with the back of a sofa. This provides a sturdy side, which also restricts entrance to one side. The downside is ignorant people jumping up and down on the sofa knocking your fort down, but such is life.
If your room happens to contain a grand piano, then you're in luck. You can tuck the edges of the blanket under the piano lid and use the piano bench as a tunnel for people to enter in. The downside is only restricted by your height compared to the space under the piano.
Using sheets instead of blankets allows quite a bit more light inside the fort. However, the sacrifice is that your fort become less of a fort, and more of a tent made of chairs and sheets, but if you're fine with that, and would rather have natural filtered light to read, play games, or make plans for when the world stops ending instead of a flashlight, then go for it. WARNING: however much the temptation is, do not hammer in spikes, tacks or nails to anchor your TMoCaS to the floor, no matter how many times on TV or in real life you have seen people do this in the outdoors.
Abandoned fort etiquette:
If you become bored, or the fort falls apart yet again and you just can't be bothered to put it up again, or an exuberant person or pet leaps on the fort to collapse it, you are allowed to leave the fort as it is and carry on with any other activity elsewhere. There is an historically accurate tradition of forts made of logs, bricks and stone abandoned due to disease, starvation, invasion and boredom, and many of them are still standing today. You may have to scavenge the blankets later on in the evening, to use for making your bed and sleeping in it, too.