Not easy or cheap; Someone correct me if I'm wrong, but just about all cars today, the electric windows vs manual windows are an entirely different assembly that comes whole, not just a simple drive unit that snaps in.
The only practical way to do it is too find a junkyard donor car, that the owner is willing to allow you to do a swap at a reasonable cost. You can get all the extra different pieces as well, for a reasonable cost, like the door skins, crank handles and inserts where the switches went. You'll probably have to drill out some large pop rivets and then pop in new rivets during the swap.
I wouldn't be surprised if only the glass is the common part, even the rails and frame the glass rides up and down on are often different for manual/electric on most cars.