Rennes to Fougeres

I took that bus in 2017, but I don't remember any details. My normal procedure is to go to the bus station (Gare Routiere) and board there so I don't have to worry about finding the correct, random bus stop along a road, identifying an approaching bus and flagging it down. When you board at the bus station, you normally are expected to have a ticket in your hand when you get on the bus; there will be a ticket window somewhere. If you board at another stop, you usually pay the driver. I cannot promise that there are no exceptions to those rules. Bus drivers seem to sell tickets just for the current run, not round-trip tickets.

Bus schedule Rennes-Fougeres

The schedule shows multiple stops in both Rennes and Fougeres. You can use Google Maps to find their approximate location, but I feel more comfortable going to the bus station. Every now and then there's a temporary change in the route that could leave you standing at an unserviced bus stop.

Fourgeres is a lovely town with a fascinating castle, so you're making a good choice. Have you considered renting a car for the day? It would be more expensive but also much more convenient than taking the bus. There are many agencies at the Rennes train station and getting out of the city isn't too bad from there.