Personally I just make sure there are no tobacco bits on the rim or lid and screw them on. The majority of mine have also formed their own vacuum. I also periodically check them and snug up any that have come loose and no problems to date.
Also, if you don't heat them, as the magical tobacco aging bugs use up the oxygen and turn it into aged magical goodness, it will form a seal on its own. No real need to heat them, unless you are just a worrier.
Making sure the seal doesn't have any tobacco in between the rim and jar lid is about the only thing you have to do.
I never heat them and after about 6 months they pop if you open them. Also, I use small jars 4 Fl oz. they hold about an ounce of loosely packed tobacco.
This lets me smoke a variety of open blends while still letting some of it continue to age.
After filling the mason jar and wiping the rim of tobacco dust, I will often run a damp rag over the lid seal before placing on the jar in hopes that the wet rim gives a better seal. But this may not be necessary.