The first step is to choose the question. In your case, you want the character question. These can come in a number of forms. You can get a :
b)How does Steinbeck use character x to present aspects of life in 1930s America?
b)Statement question e.g.: "Most of the characters in OMAM lack the strength of character to change their lives" To what extent do you agree with this statement?
b) Write in character as character x?
And a number of other forms.
I usually recommend taking the statement question if you can. If not, turn the question into a statement and then write it at the top of the page e.g. "Steinbeck uses the character of Crooks in a number of ways to illustrate mainly negative sides of 1930s society.". This is the one-liner to start your essay. You can try to make it clever and catchy but the most simple are statements as above.
Now, from 2015, 66% of the marks are for context. It doesn't matter if it's a character question, they want to hear about 30s America. That doesn't mean they want paragraphs full of detailed historical analysis. No, all they want is to see a few links to show that you understand that the book was set in a different time.
Then, think of what the question is asking you. And read it over a few times. Think of a few key points in the book that you could talk about and a few quotes you could use in the essay. Write about 5 main points you can talk about. So, for the Crooks essay, we might list: Squalid living conditions, Racism, Loneliness, Lynching and Powerlessness as aspects of 1930s America.
Then, make sure you can back up each point with evidence from the text. There's no point saying "Crooks is used to illustrate loneliness in 1930s America" if you don't have a quote and a little clever something you can say about the quote to show that you understand its meaning. Here, a good quote would be "a guy gets too lonely and he gets sick" or “S’pose you didn’t have nobody?”. Then delve into the quote a little more. Why did you pick this quote? What interesting words are there? What do they tell us about the character? Why did Steinbeck use them? How do they make you feel?. So, I might say
"Crooks has suffered a life of solitude and segregation which was commonplace in the darkness of 30s America. When Crooks says "s'pose you didn't have nobody", he is almost bullying Lennie and his face "lights" with pleasure when he does so. This suggests that he is very jealous of George and Lennie's relationship and shows he is very lonely. The way he gets mean to Lennie is, in my opinion, not a sign of maliciousness but rather evidence that Crooks is "sick" of being alone and "not allowed to go in the bunkhouse". He tries to make Lennie understand what true loneliness is like. In 1930's America, segregation was commonplace due to controversial 'Jim Crow' laws. This would have contributed significantly to Crooks' loneliness"
Here, a point is made. A few quotes picked out and discussed to some extent. Context is then brought in. This is a bare bones paragraph and really nothing exciting is said. To get marks, you have to say clever things and look at things from different angles rather than regurgitating a revision guide. Pick a few quotes that are more abstract or analyse character or images in new and creative ways.
That's kind of it. Good luck