Error pattern analysis starts with the teacher or editor identifying errors.
If at all possible, make a photocopy of the piece of writing. You need to have a copy you can mark up, and it's disconcerting for writers to see all the markings you'll make as you get started with the analysis.
Read through a paragraph or a page without marking anything just to get a feel for the number and range of errors. Depending on the number, decide if you can mark all errors in one pass or if you'll need multiple passes.
Go through the first paragraph slowly, marking as many errors as you can find. Ignore stylistic choices but note word choices that are wrong. Ditto with punctuation.
Try a sample. Keep working with a sample through all the steps we outline in the module.