You don't. The word "tar" is a slang term for a painted haversack. It was a mixure of linseed oil, lamp black, and a few other items. Getting the formula right is not easy, and can result in really messing it up, and not drying. I recommend you just buy a proper one. Some dealers use synthenic treatments, because the period version have some harsh chemicals, and can be flamable. Others have prefected it to a art form.
S.D.Swart