Shorten, shorten, shorten, and rewrite.
When writing directly for the thesis, people often are elaborate and generous. You have all the space in the world, and having 180 pages instead of 150 might be a plus.
In a journal paper you have, say, 10 pages of a condensed formatting and that's it. So, the writing has to accommodate this.
I have three suggestions:
- In most cases you would like to pick some aspect of the thesis, not the whole work. So start from the relevant chapter.
- It depends a lot on the writing style and personal preferences, but for me personally it's often easier to read a paragraph and to come up with a way to tell this in one sentence (i.e., rewriting) than to edit said paragraph to be shorter and more concise.
- All things that are somewhat relevant, but too large or not immediately needed either land in the supplementary document, or you directly refer to the thesis for more details.