"To use sage tea for decreasing milk supply, infuse 1 tablespoon of dried sage in 1 cup of boiling water (or 20g dried sage in 50 ml boiling water). Steep for 5-15 minutes. Drink 1 cup, 2 – 6 times per day." (source - kellymom.com)
As for if you can use your milk, I can't find anything that would suggest that you can't, but that goes with the obvious disclaimer that I'm not a medical/herbal specialist.