To electrify a way, select an electrification tool in a waytype's toolbar, click on two points on an existing way (you can use click and drag like ways'construction) and the way between the two points will be electrified :
The keyboard shortcut “e” allows to us quickly the last electrification used.
Like ways, bridges and tunnels, electrifications have a maximum speed, take care of it before electrifying a way if several electrifications are possible.
Next tutorial : Trams