This is more for my own reference, as my baby beetroot are finally ready!
Wash the roots being careful not to break the skins. Trim the tops to remove any leaf residue but do not cut back into the flesh. Cut off the root to leave a tiny "tail". Place in sufficient boiling salted water to cover the roots properly. Bring back to the boil and simmer gently for 1 to 1.5 hrs depending on the size. When fully cooked, a knife should sink into the flesh easily. An alternative is to cook at 15lb pressure in a pressure cooker using the manufacturers instructions. Allow to cool, skin and trim. Cut the main root into rounds about 3mm-6mm thick depending on your preference. If the rounds are very large, cut each one into 4 pieces so as to get them into the storage container easily.
If the pickled beetroot is to be used within a week or so (and stored in a fridge during that time), the rounds can simply be packed into a jar and covered with cold malt or pickling vinegar. If they are to be stored for a long time they should be packed (not too tightly) into proper pickling or bottling jars and covered with boiling vinegar and sealed immediately. Modern jam jars that create a vacuum seal can be used but make sure that the lid is clean, the seal is good and that the vacuum has been created (the central dimple should be depressed.)
With all pickling and preserving, cleanliness is important so make sure that the jars are very clean before use. When bulk pickling, we have held our jars at 120°C. in the oven for 30 minutes after washing, just to be sure.