Depending on how dry the tobacco, I either:
Spray a wee bit of distilled water directly on it, then let it sit a sealed glass jar for a few days. I keep this process going until I am happy with the moisture level, or
Spary a wee bit of distilled water directly on it; then put it and a sandwich baggie containing a wet washcloth into the sealed glass jar, adding moisture to the washcloth until I get the wanted results.
I utilized the latter method to "restore" three pouches of Troost Slices (found in the back of a drawer!) last year. Took me six months of slowly introducing moisture; however, the results were outstanding!!! Excellent tobacco! FTRPLT