Use cliches. Introduce flat characters and improbable dialogue. Just write.
Make stupid mistakes and ignorant assumptions that you later kick yourself for. Don't show it to anyone; just write it. Write something so gratuitously self-indulgent and long-winded that you wouldn't force even your worst enemy to endure the whole thing. Then put it in a drawer for awhile -- a day, a week, six months -- and come back to it.
Sometimes you will surprise yourself and find that it wasn't as bad as you thought.
More importantly, the more you write crap -- the more you give yourself permission to write crap -- the more you will write. And the more you write, the better you will get at writing, and the less crappy your crappy first drafts will be. Whereas, if you are terrified of making mistakes and not being good enough, you will never even finish that crappy first draft.
To be clear, don't stop at the crappy first draft. I do not endorse putting crappy art out into the world, particularly when you are advertising it to strangers and asking them to give you money. Revise, get feedback, revise again. Put it away for awhile, get some distance from it, then come back and revise it again. Read it out loud, then... well, you get the picture.
But, for me, the first draft is always the hardest, because I don't know what I'm doing yet, and I'm afraid it will suck.
That's when I have to remind myself, it probably will suck! You know why? Because I don't know what I'm doing yet! That's why it's called a first draft! I will make mistakes, but I can fix them later. Shut up and start writing.