How to make an air condition unit cover?

You can buy air conditioning covers at Home Depot or Lowes. You can also use a piece of canvas and just wrap a bungie-chord or rope around it.

What are you asking? Where to buy one? Homedepot or Lowe's, or any hardware store. Good Luck!

Is the unit outside as one unit or inside in a window?

I just had a new central unit installed. the installer told me NOT to cover the whole unit because it traps moisture in it... His advise was to only cover the top of the unit with a piece of plywood and weight it down with something heavy to keep debris out. Window units you can wrap to keep drafts from entering.