Question:

To provide more details, should you flush out or flesh out your plan?

Answer:

flesh out

How to remember it:

Think of fleshing out a skeleton. To flesh out something is to give it substance, or to make it fuller or more nearly complete.

To flush out something is to cause it to leave a hiding place, e.g., "The birds were flushed out of the tree." It can also be used figuratively, as in "flush out the truth."