The hardest part of timeboxing is cutting out the parts that you were hoping to do, or that you Ought To Do.
That is: The part where you have to accept the quality you can produce in, say, 15 minutes, instead of letting the task balloon out to an hour. It's acknowledging: Yes, this paragraph is poorly thought out, and it maybe needs more detail. It's acknowledging that and then saying: And yet I only have an hour to write my three paragraphs for this project, and something is better than nothing, so let's move on to the next paragraph.
I find this tricky to do. When I timebox, I still notice all the shortcomings in what I'm writing. There are transitions that need to be filled in. There are challenges that need to be considered, acknowledged, and discussed. There are important questions that aren't answered. There are sources I could quote or cite. For each source cited, I could take the time to dig up the relevant part and reference it directly. In 15 minutes, I can only do so much. I must prioritize. I must decide, for a lot of these, that I'm not going to address them, because I have only so much time.
But I can do it. I did it multiple times last week: With 3 paragraphs I needed to write for a work project, with a choice I needed to make for a side project, and with a fun worldbuilding project. And I just exercised it again this week: Read as much of an interesting paper as I can, and leave it at however much I can read.
And once I've gotten something down, I've got to be okay with "shipping" whatever I've got. That also sucks. There's always more I could say. I could always make what I have already written more precise. I can often find another tangent to elaborate on. But at some point, I have to say: "That's it. It's done."
When timeboxing, that point is whenever the timer is over—or if I'm feeling brave, sooner than that.