Yes Hollie, just use the beetroots straight from the ground - there's no need to store them first! - Snap Lisa!!
Tim, Sarsons make a ready spiced pickling vinegar which makes life easier and save the jar for making your own pickled onions! You will need to sterilse the jam jars first by giving them a good wash and then lay them down on the racks in the oven at 100c for about 20 minutes. Let them cool, then add the sliced beetroots and cover with the spiced vinegar. Using hot vinegar extends the shelf life. So you need to fill the jars while they are still warm from the oven. Cover with vinegar proof lids that have a plastic coating inside straightaway and wait a few weeks before eating and enjoying!
Ocado: Sarson's Pickling Malt Vinegar (Product Information)