I just sent them an email explaining the project, what I had done so far for the other tracks, a little background information for them (and of course asking what they'd charge haha). They were both responsive and easy to work with. I've worked with a lot of rappers on different people's stuff including my own and I thought they were really on top of it.
I used to do way more live shows and would open up for a lot of underground emcees coming through town, those connections have helped me out a lot. Like I referenced opening up for Eligh and Murs when I sent an email to Scarub and I think that gave me a little more credibility then most "collab bro?" type of peoples...