You really can make your own pickling mix but this is the general ratio that I use.  This should be enough for about 3 large bunches of cooked, sliced beets.  I usually roast the beets wrapped in foil with a bit of olive oil and salt and pepper, but you can also boil them.  If the latter, add a bit of the vitamin-rich cooking liquid to the pickle.  I usually just mix the beets with the vinegar mix and, although jarred, I just refrigerate them for relatively immediate use.  They do last, refrigerated, for quite a few weeks.  However, if you do want to preserve them, boil the pickling liquid, pack the canning jars with the cooked, sliced beets, raw onions, and any additional herbs or seasoning and then cover with the boiling liquid.  Seal with lids and caps and let cool before storing.

3 cups white vinegar

1 cup cold water

1 cup sugar

2 tablespoons salt

For each jar, optional: 1½ teaspoons pickling spice

3 small sprigs fresh dill

1 small white onion, cut, lengthwise, into thin slices