I asked the customer to give me a rating before the end of the contract so the next job will be easy to get. However, he tells me that he cannot find the rate option. I would send him a step by step guide, but I don't know where to get that information.
Both you and you client can give feedback to each other once the contract is ended. There is no option to give a feedback on an active contract unfortunately. You can communicate with your client and ask him to end the current contract, give you feedback and then rehire you again. Having good feedback on your profile will definitely improve your chances of getting hired for other jobs.
Here is more information about how the feedback system works in this article.