I would cut the return in when the water gets close to it then wait the next inch or 2 for the skimmer.
Probably be pretty close not long after u get the return all done. U can either plug the return or go ahead & hook it up If u are ready. The reason for waiting until the water is pretty close is that the water pulls the liner down so if u do it early on in the fill process the liner may pull or stretch more than the holes u cut & be pulling on those penetrations forevermore. When the water is a few inches from the bottom of the skimmer the liner is pretty much where it’s gonna stay. If u are getting close to turning the water off for the night when the water reaches the skimmer level wait until tomorrow to cut it in - incase shrinkage occurs overnight.
On another note - since it takes days to fill a large pool you need some chlorine in the water asap. You can use trichlor in a floater if u tie it off to ensure it doesn’t ever rest on the liner.
Fresh fill contains no cya so any liquid chlorine u put in won’t last long in the sun. U can use the hose stream to mix some liquid chlorine in for the night , brush the area well . 2-3 ppm should be plenty. Replenish this daily until the pool is running & u can add 30ppm cya via the sock method.
Use
PoolMath to calculate amounts based on current fill volume.
Do u have a test kit yet?
Test Kits Compared
If not, here’s info on how to get one in Canada

Someone recently mentioned being able to get the Taylor k2006 on Amazon.ca
www.troublefreepool.com