The ideal scenario is to just write when you get an idea. Don’t worry about typos, don’t worry about mistakes, in fact don’t worry about anything. Just write, allow thoughts to freely flow from your mind.
Later read over your work and start the polishing process. Generally writers read over their work three times. The first time to correct word choice and sentence structure; the second time to fix mistakes and typos; and a third time to see that nothing was missed.
Writing is like telling a story or delivering information. When you speak to someone, you do not constantly interrupt yourself to change words or statements, so neither should you do this while writing.
You should preferably not edit your work straight after it is written. In the event that you could not stop yourself and performed a first edit, a second edit should be done after some time has passed. You should return to your work with a fresh mind and alert eyes.
In the process do not be overly critical about your work. Initial thoughts and expressions are often the best ones. By changing sentences and paragraphs over and over again, your work might lose its spontaneity.
Editing should not be done in a busy environment. Television, music and conversations are all distractions that have a negative effect on the mind.
Neither should editing be done in a hurry. When you have taken the time to research a topic and have carefully constructed your work, enough time should be allowed for the editing process. A job done in haste is usually a job not well done.
In addition, the editing process should not be performed when you are tired, angry or frustrated. A beautiful piece of work is often plucked to pieces because of emotions.
Finally, at some point you have to be able to let go. You need to be able to accept that your work will never be perfect. Chances are that the reader will not read your text word for word. He might glance at certain parts, or look for information and once he found what he is looking for skip the rest of the info.
Have confidence in your writing. It might be better than you think.