What kind of dye and how to dye leather?

I have a black leather boot that needed some repair. The shoe cobbler fixed it and put a gray patch over it to reinforce it. I thought she was going to dye the patch to match, but she didn't, so I will. What kind of dye is best for leather? Is there a particular brand I should use? And what are the steps? I know it's not like dyeing anything else. Thanks.