Editing, then, is something which takes place throughout the writing process. You might make small edits as you go or make bigger changes and move sections around. Even once you think a piece of writing is finished you might come back to it and make further changes. A piece of writing often ends up looking very different from how it began.
Editing at the start of the writing process can look very different to the editing which takes place later on. As you begin writing you try to express your ideas, organising them and putting them together under different section headings. The writing is rough because you need to work through what you think, what your argument is, which theoretical work you will drawn on, and how it is going to be organised. Don't be scared of a very messy page. There might be notes, unfinished sentences, quotations to inspire you, half thoughts. This is all perfectly natural and will provide the raw material for your later, more polished work.
Once you have tidied this up into a rough draft, you can zoom out and look at the overall shape of the writing. How does it look on the page? Then read it, possibly aloud. Check that you have a clear argument. Check that each section moves the argument forward. Does it stay on topic? It's a good idea to re-read the assignment brief and the mark scheme and check that you're addressing what you've been asked to do. In fact, keep the assignment question or task at the top of the document that you are working on and keep checking it. It can be really nice to echo the words from the brief in your writing to show the reader really explicitly that you are answering the question.
You can then move on to look more closely at individual sections. Do you clearly signpost to your reader what is coming up? Check that paragraphs make a clear point and include the relevant evidence. Check that you have written critically rather than descriptively. This means that you are making thoughtful, analytical, evaluative comments about what you have read rather than describing the content. You might need to re-write sections and move things around. This, of course, means that you will want to re-read the whole piece of work to check that it still flows and then make any necessary edits.
The editing process is time consuming so it good to plan ahead and allow time to reflect on your writing, and come back to it after a break.