Reefing

My first thought about reefing was the ties around the boom along the foot of the sail, that are each tied with a reef knot, but these need to the the last thought, not the first.
At either end of this line of reef points is a cringle, a ring in the sale, one on the luff, that becomes the new tack, and one on the leech, that becomes the new clew.
First, obviously, lower the main sail partially, then using a length of thin rope, tie the cringle in the luff (nearest the mast) to the boom.
Then using another length, pass the rope between the cringle on the leech (side furthest from the mast) and the clew, getting as tight as possible, then, at the cringle, tie the rope tight round the boom.
Once the tack and the clew are resolved, then the ties along the boom can be tied, using a series of reef knots.