We commission from people who build WordPress sites rather than from people who write about building them.
What works
One question, answerable in a thousand words, that somebody would actually type into a search box. A tour of a whole subject does not.
What to send
Two or three lines on the idea, a line on why you are the person to write it, and a link to something you have written or built before.
What happens next
You will hear back either way. If it is commissioned, the rate and the deadline are agreed before you start writing.