What makes you think that your hostas are overcrowded or rootbound? Are they in containers or in the ground? If they are in the ground they can go for several years before becoming overcrowded. Anyway, you can either dig and divide them now or wait until early spring to do the job. It is somewhat easier in spring as there is no foliage to get in the way. If you do it now, cut down the leaves prior to dividing so that you can see what you are doing. If it is dry, water the plant thoroughly the night before. Dig round the outside of the clump deeply and then pry it out of the ground. This is a heavy job if the clump is large, but you want to lift a good-sized root ball. Heave the clump onto a big piece of tarpaulin or burlap and then cut it into smaller sections. You can't avoid cuting roots so don't worry about that. A sharp spade does a good job or you can insert two spading forks back to back and pry the roots apart that way. It is wise to make the pieces large enough so each has several growing points. In the spring, look for several new shoots coming up. Replant the young divisions right away at least 18" to 24" apart and water deeply.