A fascinating read on making simple syrup that is shelf-stable, and some surprisingly fascinating food safety lessons: http://cocktailjen.blogspot.com/2007/04/not-so-simple-syrup.html

In short:

Stir sugar into water in a 1:1 ratio
bring to boil
Add a dash of cream of tartar
simmer for 15 minutes
Let cool. If it hardens, add more water, repeat.

You now have shelf-stable simple syrup.

Add cranberries: 1/2 bag (1.5 cups) during the simmering process, let drain out.

Add mint: 15 cups packed fresh mint, drain.

Add a shot of neutral spirit (vodka, etc) to help preserve these syrups.