You start in the village Røldal and finish in Kinsarvik.
Røldal can be reached with a direct bus from Oslo in just under 6 hours. (NW180, more info at Nor-way, a reservation is needed.
From Kinsarvik you can take a local bus to Vossevangen. Reservations are not required, download the Skyss Billett app for tickets. From Vossevangen you'll have regular trains to Oslo. (F4, more info at Vy, a reservation is needed).
The route
Hereunder you'll find a digital map. You will also find a GPX track on top of the page.
Water
You should have nog issues finding water. There's plenty of mountain rivers providing fresh clean water.
Food
As the route is designed to avoid busy spots, you'll need to take all the food you need with you.