Since the fast has already begun, one cannot make Havdala on Motzai Shabbat, this week.
What one does, once it's Motzai Shabbat:
- Before removing one's shoes and doing any work one says בָּרוּךְ הַמַּבְדִּיל בֵּין קֹדֶשׁ לְחֹל.
The Kitzur recommends one says the longer version:
Thursday, 7 Av 5775