Turn Quality Into Its Own Step

Demanding quality as you compose causes the ‘writing’ and ‘editing’ subtasks of a project to interfere with each other in an agonizing way.

— James Horton, The Nonwriter's Guide to Writing A Lot

Force yourself to separate the two subtasks by deliberately writing poorly.

You can write poorly by taking on some persona that you know would not write well (someone on drugs or a fictional character).

You could also change the medium that you are writing something in like making it an email rather than an essay at first.

Or you could overwrite. Write as your mind goes and spend no time trying to stop yourself from using too many words. You can come back and edit later.