The opposite of writing is not writing, which seems obvious except when you're staring at a blank piece of paper or computer screen, thinking about what words to choose. That is thinking, or ruminating, or perhaps even procrastinating.
There may be valid reasons for engaging in such behavior. For example, you may need to think about a thing before you start writing. Very well, then: think. Do that thing instead, that one thing.
But when it comes time to translate those thoughts into readable symbols, focus all of your energy on that activity. Put the words on a real or virtual page and just go, until it's time to do something else. Then stop and move to that other thing, focusing all of your energy on it instead.
It's harder than it sounds. Like walking. Sure, it seems easy now, but how many reps have you had? Was it easy when you first started, or did you fall over a lot before mastering the movements? If you aren't sure, maybe ask your parents. They probably have some funny stories about that.
Okay, so you have words. That's all you need. The trick is using the right ones and arranging them in the right order. Planning is involved, though it's always helpful if you're ready to abandon any plans in favor of a better one should it arise during the actual construction.
Once the building commences, you may find yourself surprised at what takes shape. So flexibility is a good skill to have. If what you have looks different than what you'd envisioned, either push in the direction of your original vision or modify the vision itself.
You always have choices. You can always clean things up later. Like sand castles, which are always washed into the sea, you can make others.