The easiest way to cellar tobacco that is NOT already sealed in a tin, is by sealing it in a mason jar. However, the seal on a mason jar CAN fail! There's no point in storing a jar of baccy for decades, and then finding out all you have left for your effort is a pile of dry tobacco dust.
Firstly, the term "mason jar" is applied to the SCREW TOP jars on this blog. There are also "bail top" mason jars, these have a spring loaded hinged lid that presses down on an almost comically large rubber washer. Bail-tops are NOT ideal for long term storage, they are fine (in fact, great) for short-term use. Their ease of opening makes it a quick task to pop open, load a pipe, and close back up. The wire springs loose tension over time, and the rubber gasket will also distort and could form gaps, allowing air leaks.
The screw-tops are the best for long term cellaring. These lids come in two parts, the LID and the RING. The ring simple tightens down the lid over the mouth of the jar. Most lids have a pop-up "nipple", this is not really a tamper-evident function, but an air-tightness alert. The rubber washer is thin on these lids, and soft. When jammed down, the rubber forms a tight seal by getting distorted and mashed down, effectively filling in all microscopic gaps. Once you open it, this seal starts to deteriorate. While fine for keeping things going as you combust the jar contents, avoid re-using lids. Danged things are less than a buck each - just get new lids (and mostly they come with matching rings)!
The pop-up button will only work when air is evacuated. In "canning" of foodstuff, the jar is boiled or cooked before the lid is put on, this process removes all air. With pipe tobacco we don't really want to cook the precious cargo. One good recommendation is to run the (new) jar through a dishwasher cycle - NO detergent, and with heated dry. Lids should NOT be washed. The dishwasher will wash icky things out of the glass jar, and the heater will dry it out as well as make the glass hot. If you quickly load in the tobacco, not allowing the jar to cool, then put the lid on and tighten - it will cause the popup button to be pulled in as the jar cools, and cools the air within, and reduces its volume thusly. As long as the seal holds, the button stays down. An easy way to judge the health of your cellared tobacco!
It is suggested that you leave a quarter inch or more of space at the top of the jar. Especially if you are packing (loose cut) baccy into the jar, as it will start to expand and could push up against the lid, possibly breaking the seal. And a wee bit of air is necessary for proper aging in any case. Another thing to watch out for are loose strands of baccy that could get between the mouth of the jar and the rubber on the lid. Leaving sufficient air space makes it easier to avoid this simple error, which leaves a nice air leak channel through the rubber seal. Periodically checking your jar collection for up-turned nipples is a good practice.