A pitch should show that the story exists beyond its theme. Instead of proposing an essay about repair, introduce the repairer, object, archive, or conflict that lets a reader encounter the subject in a particular way.
Three or four considered paragraphs are usually enough. Include the central question, what you already know, who you can speak with, what remains to be reported, and why you are suited to the assignment.
If a pitch interests us, an editor will ask questions before offering a commission. Together you will agree the angle, approximate length, fee, expenses, deadline, and any photography or illustration. The commissioning email records those terms.
During reporting, tell the editor early if access changes or the evidence points toward a different story. A strong commission can change shape; it should not change shape in silence.