Editing is a strange thing. There are a couple of different types as far as i can tell. Editing your content, which is where you look for plot holes and structural changes to the story that will make it stronger - or checking for grammar and spelling.
The first bit of advice I can give is - unless you have finished your book, don't try and edit it (unless there is something specific you need to change - then yeah - do that before you forget) - but don't keep reading through the beginning - you will absolutely keep finding things and get pulled away from the part of the story that you are actually on. There are things that I put into the beginning of the story after I had finished writing the book, because there were things I knew at the end that I hadn't known when I started writing the book. I don't always know every detail before I start a book (obviously or the writing would be easy) and so i like to give myself the time and distance before editing for plot flaws - because the closer I am - the less I see.
The other kind of editing is more difficult for me, I don't know what an adverb is, my punctuation is shit and I forget to format paragraphs properly. I read through it on the computer many many times - I don't think i would be lying if i said i had read through it AT LEAST 50 times on the computer - fortunately i am a fast reader and so it only takes me a day to read it through. That was great. I could do spell check over and over and then still find errors. I had a reader read my book, and she (i assume it was a she) said some of my dialogue was a bit hammy (how dare she!) and so i printed off my book and then went through the whole thing with a highlighter pen and highlighted every single bit of dialogue. I looked through the conversations and saw if there was anything i could fine tune. It was actually an interesting exercise to see how much dialogue I had in the book, I think I had a fairly good balance.
I also went through wherever I found a mistake and put a post it note on there, colour coded, pink for spelling, green for typos, orange for plot relating to the sequel, yellow for grammar..etc
Finally there's formatting, paragraph indentations, proper line spacing, making sure you have page numbers - also the title of your book at the top left header and the author name the top right corner. Capital letters where they should be and stuff like that. I also post it noted areas where that was wrong.
the key is organisation of the mind, and also just removing the emotion from it. its a product, you wouldnt try and sell a jumper you had knitted if there were holes in it would you? who would buy it? An agent needs you to make their 15%, so they are not going to represent someone who is submitting a jumper full of holes are they? They then have to fix the holes and move on to showing the jumper to a publisher who will then go - why would I want a jumper like that? (My analogy is thin - just go with it)
The funny thing about me is that if there is even remotely one spelling mistake in a book I am reading i totally notice it - in my own work, not so much.
That's just how I work. I couldn't afford an editor and so I had to rely on myself mostly to work it out, like everything with my writing. I am self taught, I didn't go to University and do a masters degree in creative writing, (in fact i did six months of a computer software engineering degree before realising I was definitely a creative type and giving up) - There's nothing wrong with doing a degree, but I believe with the right amount of dedication and determination you can learn without it. Also I suck at education.
Remember, its important to know the rules so that you know when its OK to break them.
Until next time
also - heres a great mash up video for inspiration