When one hits this last of the three stages, students are usually full of anxiety. We've gone through all the planning, the preparation, and now it's the time to actually go through the only step that produces a result. And it's that anxiety that causes us to mess up.
Instead, we once again have to go back to the production line mentality. The one executing the mark is not the one who planned, nor the one who prepared. They're a new version of ourselves who was given simple instructions, and only needs to worry about one thing:
Executing the mark with confidence. No hesitation, no second-guessing, no attempting to avoid a mistake. If you make the mark confidently but mess up on accuracy, then you didn't mess up. It's one of the earlier people up the production line that messed up.
Your job is to make a single smooth stroke. The second your pen touches the page, you need to accept that any opportunity to avoid a mistake has passed, and all you can do is commit to the motion and push through.
That's all there is to it.