In order to send one of your Sicarios to defend your Base, open the list of your Sicarios by tapping on the special gun mark on your main Base screen (in the lower left). Then just tap on the card with the strongest defensive Sicario and there you go!
Depending on their level, players may have up to 6 Sicarios defending their Bases.
However, this is not all! You may join a Cartel with the Bolster Defense research completed and put up to 8 Sicarios on your Base!