Reccomendation letter

I've been asked to write a letter of reccomendation for my professor who is a candidate for a job renewal at my undergraduate college. I'm honestly stumped as to how to go about doing this! I don't how to start, what to put in the middle, how I should head it, how I should end it etc. He was an amazing English/Creative Writing professor and I'd like to do the most I can to help him - especially since I was one of the students he selected to write for him. Is there any sort of guideline I can follow? What do I do??