Fast and easy until a sequence of falls: Rhaedr y Pwll Du, or "The Falls of the Black Pool". After this the water eases, but keep an eye out for wires across the river.
Takeout at Esgairgeiliog bridge just off the A487 about a km south of Corris. To reach the putin head towards Aberllefenni from Corris along small roads. Stay right after passing through the village and you'll end up at a bridge over the river.
Rainchasers is best experienced as an app.