What I did was get a full rear wheel conversion kit, which comes with the motor speed controller, throttle control, most of them come with some kind of pedal assist sensors, and obviously the hub motor and rear wheel assembly. I already had a bicycle lying around. These are almost always bring-your-own battery affairs. Everything else in the kit just plugs together, and you get to decide where to route the wires down your bike frame. Maybe bring some zip ties. E-bike battery kits are readily available on Amazon or eBay, or you can have a go at making your own battery pack out of bare cells. I was lazy, so I got a premade battery pack. The only real DIY aspects I had to deal with was mating the battery pack connector to the power input on the motor speed controller box, which required some soldering (or you could use crimp connectors, I guess) and also figuring out where to mount the battery pack and motor controller to the frame of the bike.
I also decided to make my life difficult by wiring alongside this an entire secondary 12 volt system to run brake and tail lights, turn signals, a headlight, and a horn from a car (!) because if anyone runs me over they’ll have no excuses. But you could easily omit all of that nonsense and deal with a lot fewer wires in your life.
I bought all of my conversion parts from Amazon. My buying strategy was just to look for stuff that had non-shill looking reviews.