I took my first stab at dyeing leather the other night. I tested my technique on a scrap piece and it came out nice. The dye seemed to work best when the piece was a little damp. Fill a spray bottle with water, and mist the leather until it is damp to the touch. Wipe off any exceess water with a rag, then apply the leather dye. Wipe down the leather with a dry rag, and check your color. Add another application of dye as needed. Once the piece dries, it will be a little lighter than the color when wet, so adjust the amount of dye accordingly.
No. When I picked up the leather at the Tandy store, the sales guy did not mention deglazer, and I did not know to ask. He did give me the water tip and a few other tips on creasing, and cutting, so I guess he knows about how to work and dye leather. But he did not say anything about deglazer.