Go to Rotterdam first from wherever you are. Take the metro to Spijkenisse and get off at Spijkenisse Centrum (about 30 mins).
Take the bus to Brielle at Spijkenisse centrum.
Either Connexxion 103 direction Brielle or Connexxion 113 direction Rockanje. The bus from Spijkenisse to Brielle takes about 20 minutes. So Brielle from Rotterdam it's less than an hour by public transport
From anywhere in The Netherlands, get on the motorway A15, follow Europoort. Right at the end of the A15, it turns into N15, through a tunnel called Thomassentunnel. Follow N57 direction Brielle, Hellevoetsluis, Middelburg. Turn left at the first traffic light and presto!
20 minutes from Rotterdam, 1 hour 10 minutes from Amsterdam, or 10 minutes if you get off the ferry from Hull (UK) to Europoort.