Put them int a tank, get a filter and a heater. Put in substrate and live plants. Get a light, plants need light and nutrients. The bettas will love you for doing this. I would suggest using a 2.5 or a 5.5 gal tank however.
Bear in mind that bettas are labyrinth fish which means they gulp air at the surface. Therefore be sure to have a lid on the tank and to leave some open space between the water surface and the lid. The lid will insure the air is warm and moist when they do this.
Have a read here http://seriouslyfish.com/species/Betta-splendens