You meditate on the lake.
If you don't know where the lake is then you need to progress the main story.
There's a sequence of events, you may have missed one along the way.
* Check the manor's garden.
* Take a train ride.
* Talk with Gramps
* Do martial training
* More talking
* Fetch something from the lake (check 3 times)
* Even more talking and now main story stuff.