I looked up a few pictures on Pinterest but most of the links sent me nowhere so I experimented! I baked a few small cakes in jars and then a flat cake to cut and layer in a clean jar. Process #2 was the winner. When you bake the cake right in the jar it's easy but you can't layer it and it pulls away from the sides and looks pretty messy. It still taste fabulous though. Here's what I did:
Bake a box cake or your own recipe
Let your sheet cake cool
Take the tin from a wide mouth mason lid and lay it on cake layer
(You can add a dab of frosting to make it stick if needed)
Cut around it with a knife
Scoop it out and put on a plate
Trim the sides just a bit to shrink, you don't have to be perfect
It should slide right into your jar
Add a frosting layer
Repeat cake and frosting layers
Cap and decorate.
I added my frosting layer by putting store-bought topping in a ziplock bag and nipping off the corner to make a disposable pastry bag. My decor choice was chevron tape from Paper Source and red bakers twine. Now that was doable!