Start with any writing: a paragraph from whatever you are currently reading, a poem, a bit from your favorite novel, a dialogue from a screenplay, the most recent email you sent or received.
Collect information about the text. For example:
How many sentences does it contain?
How many words?
How many commas, exclamation points, question marks, or other punctuation?
How many words (or letters) did the longest line contain, and how many the shortest?
What were the most and least frequently used words, and how many times were the most frequent repeated?
How many one-syllable words?
How many two-, three-, or four-syllable words?
Analyze the grammar: how many verbs, prepositions, nouns, pronouns, etc?
This process is potentially endless. Continue to gather information for as long as you like. The more information you gather, the more constrained the writing process will be.
Because now you will write a new paragraph/dialogue/poem that matches the information you collected, but with entirely other content.
You might find it nice to work with one parameter at a time, and create several variations.
Perhaps the first replica simply replaces each word with another word of the same number of letters. Or in which each word begins with the same letter as the original word, in the right order. Or both the same number of letters and the same starting letter.
And then perhaps the next replica will replace each word with another word of the same grammatical part of speech.
If you are inclined, make a replica that obeys more complex patterns found in the original text: working backwards, working with every other word, or with pairs or triplets of words or letters, with medians or means or modes.
If you go on like this, in two or three weeks you will have exhausted all of the obvious possibilities. And then you may choose a new starting text.