What advice would you give to George about handling Lennie in Of Mice and Men?

If I were to write a letter to George about what to do with Lennie, it would be under the presumption that the ending has not happened yet.  I would stress to George that the initial fears that both Lennie and George share about working on the ranch are to be understood and upon which action needs to be based.  Both characters express how there are some bad elements on the ranch.  George and Lennie both recognize this in Curley and Curley's wife.  I would write to him that being able to act in a manner of what should be, as opposed to what is has to be embraced.  I think that George sees what is as the only potential course of action.  The life that both characters live in terms of being migrant workers makes options limited.  Yet, I think that they could find a different farm upon which to work.  They might have to face different demons in their quest, but at least they would not be the specific ones such as Curley's wife or Curley himself.  I would advise George that the need to leave immediately is something to be respected and followed.  If the letter was timed at the point when the mob is out to get Lennie, I would probably advise George to recall the time when both characters hid out in the ditch outside of Weed.  They have been there before, why not hide again?  The accusation of rape and the murder are similar in that both characters need to hide from the social orders that are pursuing them.  I would stress to George that taking a life has profound consequences and if he can pursue an avenue where this can be avoided, his own moral compass might benefit greatly from it.

A letter from Curley is going to highlight the "surface" approach he takes towards living.

Curley is a "surface" character. He lacks the depth of Slim and lacks the understanding of someone like Candy. He is the boss's son and enjoys the privileges that go with it. As a result, much of what he would write would focus on what he immediately perceives. The letter would lack much in way of emotional depth because this is something that Curley himself lacks.

A letter could focus on how Curley is distrustful of the new arrivals.His letter could address how George gave evasive answers in their first encounter. Curley could not understand why George and Lennie travel around together, evident when he remarks "So, it's like that, huh?" George held his own against Curley, and refused to capitulate to begging and deferential respect. Curley might write about how he saw this as a direct threat. Given how Curley does not like "big guys," his letter might also spend some time discussing how Lennie is someone towards whom he must be vigilant. Curley could mention that Lennie did not speak when addressed. While he doesn't indicate this in the text, it is clear that Curley understands that something isn't quite right with Lennie. He could allude to this in his letter.

What advice does George give Lennie for getting along with Curley in Of Mice and Men?

George learns very quickly that Curley is the boss's son and a bully. He is short and has been a lightweight boxer. He doesn't like men who are bigger than he is, like Lennie.

When Curley first meets Lennie, he bullies him because Lennie lets George talk for him. (George warned Lennie beforehand to keep his mouth shut.) Curley orders Lennie to talk for himself. Curley seems to be spoiling for a fight with Lennie.

After Curley leaves the bunkhouse, George says he's scared of the situation they are in because of Curley. He says they will get fired if Lennie gets into a fight of any sort with Curley. He advises Lennie very strongly to stay away from Curley as much as possible:

Look, Lennie. You try to keep away from him, will you? Don’t never speak to him. If he comes in here you move clear to the other side of the room.

Lennie agree to do this.

In a nutshell, he tells him to stay away from his wife, only known in the novel as Curley's wife.  Curley is excessively protective of his wife and suspicious and jealous of anyone who tries to interact with her in any way.  When Lennie expresses to George that he thinks Curley's wife is "pretty", George realizes that Lennie may get in trouble by showing any interest at all in Curley's wife, so he wants to assure that Lennie doesn't interact with her.  In addition, George feels it is best that he stays out of Curley's way because Lennie often tends to unintentionally get in trouble whenever he opens his mouth.