I put mine in by myself with both the front and rear glass still in place, over the corse of a weekend.
First time I ever did it and it turn out very nice.
Might be easier with the window out, but I can verify
that it doesnt have to be out to install a headliner.
Just go slow and easy. Dont cut anything until you are sure it needs to be cut.
Cut the hole for the dome light last. I have seen way to
many do it yourself attempts where they have a big nasty
cut or gap next to the dome light where they didnt have it fully installed and cut the dome light area out. You can "fish" the wires out after its cut.