I use my mini-mag lite mounted in a backpacking head band (which I've used packing out on the trail at night in bad weather) for working under sinks, in attics, under vehicles, etc. Converted it to LED a few years ago. Safety glasses can be helpful too if crust is falling from the old fittings.
I've been using plumber's putty to seal the plastic gaskets, although I don't know if it is necessary. I put a thin-rolled ribbon in the empty spaces, tighten, and wipe away excess if any. If I put in too much putty it causes problems sealing it which I learned the hard way years ago. I had some trouble sealing faucets to some old basins without the putty. Haven't had any leaky ones in years.
I need to get rid of a failing Delta in a bathroom soon.