Yes, listen to Slack's advice - it's fundamental.
Writing should be more like sculpture than painting. It's always better to have stuff to throw away than to try and add stuff later on. As I mentioned earlier, the creative side of you (the writer) should just 'go for it'. Write, scribble - just hammer away at that keyboard until your fingers bleed without much regard for doing things right. A lecturer of mine once said that it should be like a state of intoxication. What he meant by that, I think, is not to be at all critical about your writing or your ability to write at this creative time.
Once you have a first draft, then you can introduce the critic, the judge, the executioner. Just as you locked this side of yourself away during the creative stage, it is now time to lock away the creative part of you and get to the nitty-gritty, heartless work of re-reading (and re-reading) armed with your red pen and scalpel.
Of course, don't be hard on yourself, because just as many people find the act of creation very difficult the same applies to the act of editing. It's not easy, but it needs to be done. On a positive note, there are weirdos out there who are not only gifted editors but also find pleasure in it.
Like most things in life, it is wise to learn your craft by reading on the subject or, better still, joining a writing group or doing a course. Nothing like sharing your passion with like-minded loonies!