Cook the beets until you can pass a fork easily through them. Some people prefer to substitute lemon juice for at least part of the vinegar. My grandmother used to use sour salt. Dip your finger into the mixture until either you like the taste or you are sick of it. If you find you added already two liters of vinegar and 3 kilos of sugar you've got a problem.
Some people like to add a pinch of cloves or even ginger.
Slice the beets thinly. Slice up a few onions. Place the beets in a jar alternating layers with a thin layer of onions. Then pour the liquid over the whole thing until covered. It should sit for at least a day or two before using.