Research, research and more.

If you can keep the crank, chain and cassette, as well as rim brakes, you save on not needing those parts. So you then need shifters, front and rear derailer, A and B junction boxes, battery and a holder if internal in the seatpost, 6 or 7 cables of correct lengths. Maybe hydro cables and brake units if doing disk, plus the knowledge and tools to install a hydraulic brake system.

Lot's of YouTube videos on how, it generally really simple.

I got a partial kit from Texas Cyclesport. When you buy the kit they ask you to specify easy stuff like braze-on or clamped derailer, Regular or longer cage rear derailer, battery charger (yes you need it), and cable lengths, which you can easily ,measure using a piece of srting and then just order a bit longer, the cables can hide away.

And a PC laptop to run the E-Tube software for easy system configuration, you can also do it via BT and an iPhone/Pad, if you purchased the BT unit.