How do I rewrite prose I've written into blank verse?

As posed, this question implies poetic form and prosody is a shape into which one pours content, like making ice cubs or a bundt cake. Not so. In poems, the form and the content are identified, and to alter one means altering the other, unless one is verbally talented or lucky. However, sometimes the content will have to change because of the requirements of the form and sometimes the content will dictate the form.

If you want to say that Satan is not a truth teller, you might want to write

The Devil does not Tell the truth.

But if you have to rhyme, you must write

The Devil Is not on the level.

The two statements are not precisely the same, although they are related and close.

Blank verse is unrhymed iambic pentameter. Therefore, you must scan the prose you’ve written, see where the rhythm is closest to iambic and keep that. It needn’t be mechanically perfect, of course, and shouldn’t be, since skillful varying is part of poetic effect. You’ll have to get rid of any rhymes and after that alter the prose, trying to keep the sense as best yu can, so that it becomes iambic. It can be done; you have to have an ear.

I’m willing to do it for you but I charge $50 an iambic line, $2000 minimum payable in advance.