How can I learn to use African-American accent?

I’ve learned several accents but I haven’t been able to use the accent of African-American. I think this particular accent is unique and interesting. Normally it won’t be that hard to mimic an accent but I find it hard to do it in this particular accent. I’m referring to the one used by some people like Martin Lawrence, Eddie Murphy, Queen Latifah, Whoopi Goldberg, etc.

I wish the mods could give me a suggestion but I’ve tried to make this sound as less offensive as possible the best I could even though I didn’t see it as offensive.