I'm studying Of Mice and Men, what is it exactly you're stuck on? Planning the essay, what it means by "the way Steinbeck presents", what there is to know about Slim in general?
I'll give you a brief plan:
Intro - Who is Slim? When is he introduced? What is his overall importance?
Paragraph 1 - He has a dog, doesn't care much for its pups, was going to drown them right off, this contrasts with Lennie's bond to animals, shows Slims practicality.
Paragraph 2 - He is god-like to the ranchers, he leads them when they work, they look to him for advise, George confides in him about Weed because he feels he can trust Slim.
Paragraph 3 - Slim understands humane feelings, he doesn't show hostility to Crooks, helps him out in the stables, he comforts George after Lennie has died and is the only one to.
Conclusion - (I'll let you decide where that leaves Slim's character!)