@Anubi we did this on our own from Stavenger. We were in port from 0800-1600 and I booked an express round trip bus ticket through https://gofjords.com. We got off the boat, walked to the meeting point (Radisson Blu Hotel ~10 minute walk if I remember), then the bus took us ~35 minutes to the drop off point. Once there, it is VERY clear where the hike starts. The hike took us about 4 hours round trip-- 2.5 hours up and 1.5 hours down. Of note, we are in our 30's and in decently active shape. This is definitely not a hike for beginners, but well worth it when you get to the top! After we finished, we had enough time to grab a drink and light snack at their shop before needing to board the bus back.
All in all, this was VERY easy to do on our own, without a guide.