As always, practice makes perfect. One thing I like to do is use a website like Write or Die where you give yourself a word count goal and a specified amount of time and it does annoying things if you're not writing fast enough or don't meet your goal. I'll just word-barf my unedited thoughts no matter how badly they come out. Then I'll do this again the next day and it'll usually come out much more refined since I've already had practice writing. Then from there I'll refine it down to the best material and edit any phrases that aren't clear/accurate. Usually I do this in chunks of 750 word bursts, and I can get 200-350 quality words out of those bursts once I edit and refine them.

This is just what works for me though - you should experiment with a bunch of things and see what works for you. I think the one universal piece of advice I have is that you're never going to get everything perfect on the first draft. Trying to get everything right the first time will slow you down so much, and I find it rarely beneficial.