Gulp I have got my first essay to mark. I haven't been given any guidance on how to do it.
Look at the question, and work out what the essay ought to tell you. Does it tell you this?
Then look at the structure, is it clear, or all jumping about, can you understand the sentences etc.
Does it have any factual errors? A good essay structure has an intro, the main body and a conclusion, with facts referenced where appropriate.
I don't know if it's accessible to everyone, but I think York Uni had a webpage on marking, with some advised percentage marks, based on how well the student had understood the question, the clarity of the writing, etc.I'm guessing your college may have something similar, if only for students to refer to. Or google for marking schemes or something like that to get some ideas.
If you have to give feedback, then do it as a 'sandwich'. Some positive remarks, then any negative points, then finish on a positive note with ideas as to how it could have been better. Unless it's so bad you can't find anything positive to say!
Generally, getting the facts right is the most important thing, but an essay with correct info, but so badly written you can't easily understand it is no use, and a beautifully written essay with no real facts, or lots of errors is also no good.