Most definitely turn the power off for the drain. Draining times vary by method, I too used a submerible pump and it was done in less than an hour, using the drain plug on the tub could take a bit longer.
Cleaning the tub, depends on how dirty it is. My first refill I used a damp cloth and then used the gloss product from Leisure time. Unless you have a scum line or such you may not have to do anything this time around.
Your dealer should have given you steps to do after the tub is filled?? I put in MetalGon, adjust ph, alk, and cholrine and then wait for the temp to get just right. All in all my refill took a couple of hours.
I'm sure one the resident experts will chime in and correct me if I've misspoke.