For rethinking is what it takes to do an effective job with revision. Writers need to give the draft the third degree to see whether it delivers on its promises. Some writers eyeball the whole draft in a quick pass, skipping from paragraph to paragraph to get an overall picture. They make the big changes, then go over the draft in several more passes, each time looking for smaller and smaller details. Other writers make only a couple of passes, but in doing so they are constantly zooming in and out, big picture to detail and back again.
Revision does take practice, like any other part of the writing process.
Virtually all good writers are effective, practiced revisers. At the revising stage, writers have the chance to make up for any deficiencies in their prewriting or drafting. They can gather more source material, generate new ideas. They can throw out the whole draft and start over, if necessary. Unfortunately, many student writers are reluctant to revise in any substantial way, even those who bring their drafts to the writing center.