Buses leave from Paris 3 hours before each departure and from Beauvais Airport 20 minutes after each arrival. Buses depart from Paris at the coach park on Boulevard Pershing, accross from the Palais de Congres and near the Concorde La Fayette Hotel. The cost for a one-way ticket is 15 euros.
You can also have a look at the official Beauvais Airport shuttle page.
There is also shuttle service between the city of Beauvais and the airport. Stops include the train station, cathedral and many local hotels. The cost is 4 euros for a 48 hour pass. Tickets can be bought on the shuttle. Travel time is between 30-45 minutes. The shuttle stop at the airport is just in front of the Sixt car hire building.