Cut a lime and rub edges of glasses with it. Then dip the glasses in a plate that has either sugar or salt or a combination.
Combine tequila, guava nectar, triple sec, lime juice and agave syrup in cocktail shaker with ice. Shake for 20 seconds. Strain in a margarita glass and garnish with a sliced lime.
Notes
While I prefer the pulp which you can thaw out, passion fruit nectar or juice can be uses as well. You can also freshly squeeze some.
Use agave syrup instead of simple syrup. It takes takes less work and adds additional flavor.
Use silver tequila when making the margarita. Gold tequila can muddy the flavor.
Use crushed ice which will make the margarita colder quicker.