you gotta build a good vibe with cats first. that way they'll be more receptive to your suggestions. Sometimes its hard because an MC has his ego (think about it, that's 90% of their persona on the mic) and he might not like you telling him how to do his craft. I've had niggas tell me that I need to stick to the beats and they would worry about their rhyming.
I took their money, recorded them, and never answered their phonecalls again lol.
But seriously, once you can shoot the breeze with niggas and they cool with you its a lot easier. If its somebody you gonna be working with a lot, kick it with him for a week or two, do some sessions and then ease in there with suggestions.
When they get on the mic, give compliments but also let them know where they can improve. Like I might say something like that was almost there, you just lost your breath right around the 8th bar, lets run it again from the top but lets keep that take. Or it was perfect except you slurred that one word. Or if they fuck up and its obvious, just hit stop and say lets take it from the top. But if you just plain out not feeling that nigga, then its a lot more complicated. You gotta be diplomatic but just tell the truth. I'm not feeling that hook, I think it would work better on Beat 22, but this beat aint really working with that hook.
If niggas come with attitudes just keep your cool and say something like I'm just tryna help you sound the best you can on this track, its the same shit any other producer would do, you should be worried if I didn't say nothing... That's why the call me a producer because its my job to make sure this track comes out the best it can...
Most of the time its only rookie cats that come with attitudes. Niggas that's been in studios and been at it for a minute know that they gonna get their shit critiqued.
And sometimes its gonna be tension. that's the nature of the beast. Just gotta roll with it.
Make sure you listen to their verse FIRST, before you even get to the studio lol. That will save you a lot of time and money off the bat. Forgot to mention that one.
I got my own studio, so I always forget about that, that's prolly most important.
Way back in the day (bout 93-94) my brother n'em used to have a rap group, they would record scratch tracks at their producer's house on a 4 track tape deck using a sm57. Before they even went to the studio their shit had to be memorized and tight. I remember a couple oof times my brother had big attitudes because the producer told him his shit wasn't hot. its just part of the game.