Advice is appreciated.

Relax and put it off until Tuesday. Meanwhile, get your window installation
instructions out and read them. If you don't have any, call Andersen and get
them. Pay attention to exactly what is called for in the install, like RO
precisely 1/2" larger than the window frame, and all parts of it square plumb
and level. Gather your supplies, like roofing nails, a roll of flashing, and
especially shims, and latex foam ( not the hard expanding foam) for filling the
gaps. A triple layer of heavy PE film over the opening will keep out the
weather while you're getting organized. Good luck.

Joe