Open blister on the bottom of my foot near my big toe. What's the best thing to do?

If it is open. Then it really isn't a blister anymore, it is now a wound. Apply an antibiotic and covering. I prefer an antibiotic solution like beta dine when walking as creams can cause increased friction and maceration of skin. At night time, you can apply an antibiotic cream. (if beta dine not available, just clean off the wound with saline, or hydrogen peroxide and cover, and use the topical cream at night.
Pressure. If you have already opened the blister (i assume it is an irritation blister), then i think you should press hard and remove all the liquid (serum) from the blister, so that the covering skin lays flat, it should act as a matrix to heal that way. If concerned about infection you can spread a bit od neosporin, Bacitracin or polysporin/triple antibiotic on it.