I think black leather dye will be better than Gliptones Scuffmaster
Scuffmaster is a great product but won't have the intensity of colour to dye red leather to black
You will have to degrease the leather and use a professional leather dye
Good luck, I don't think it will be easy to get a good even finish
personally I would take it to a professional saddle-maker or leather worker or at least get them to have a look and recommend a dye and method
