How mechanically inclined are you? Your first attempt could take any amount of time. I've never done anything in my life that I didn't think I could do faster and better the second time.

"indexing" means the shifter has little steps that match the exact amount of cable pull necessary to shift the rear derailleur into the next cog. That means you can shift to the next detent and the derailleur will snick the chain onto the next cog. Naturally, you have to have matching parts, everything has to be aligned as new, and the cable adjustment has to be right for indexing to work.