A common word of the English language has approximately 11 bits of entropy. That means a 256bit passphrase (passtext?) would require 24 words.
How could one make up a text of that length which is still easy to memorize? You could write a poem. The art of writing poetry and memorizing poems is not hard to learn. It doesn't even has to rhyme. In fact, not rhyming makes it harder to crack. Have fun!
However, we assumed that all words are completely random. In natural language, some words are more likely to appear after others. Poetry also usually conforms to certain rules and structures which further reduces the entropy. This answer on linguistics.stackexchange.com comes to the conclusion that the entropy per word in a poem goes down to about 5 bits per word, so you would need to write and memorize a poem of at least 52 words to get over 256 bits of entropy.