There are many methods and opinions.
When I've acquired dried out bulk tobacco from somebody's "cellar," I've had extremely good success with the following:
I put it all in mason jars, and cut paper towel circles to be the same diameter as the mason jars (or slightly bigger). I wet those circles not under running water, nor in standing water, but by spritzing them with a little spray bottle.
(It's the only way I've found to achieve moistness without over-soaking them. It also retains their flat shape.)
I lay that circle directly on the dry tobacco, and seal the jar.
I then open the jar back up once every 24 hrs.
If after 24 hrs. the paper towel is crispy dry, I pull it out, re-spritz it, put it back in, and reseal the jar.
If the tobacco is really dried out, this will happen for several days in a row.
But when the day comes that I reach in and the paper towel disk has retained a hint of moisture, I know I'm done. I pull the paper towel out and seal the jar for storage without it.
There are probably better ways, and YMMV, but this has worked perfectly for me, in the case of ribbon, coins, and flakes.
If we are talking about a plug or a rope, this will only over-soak the exterior while the interior remains crispy. It takes a very long time at a very controlled rate to get the right moisture to the center of a plug or rope without oversoaking the outside. In those cases, there is nothing for it but to put it away in a humidor for 6+ months. (I use the "tupperdor" and silica kitty litter method.)