The festival is held at Horni Hrad castle which is about an hour and 40 minutes from Prague.
You can reach it by bus from Prague going via Karlovy Vary and we recommend downloading the IDOS app to find public transport (the app has an interface in English). Here are a couple of sample options:
From the bus stop it’s about a 20 minute walk up the hill to the castle.
There’s plenty of parking and if you want to stay in your van that’s possible, too.