The recipe is told in "parts" you can make one lime's worth or 400 limes' worth.

1 part lime juice (FRESH is best)
1 part tequila
1/2 part triple sec (or other orange liquor)
2/3 part sugar
3 parts water
Ice

Mix everything but the ice. Stir or shake until sugar is dissolved. Pour over ice. Drink up.

NEVER use sweet 'n' sour mix. Blech. Fresh lime juice is the way to go for on the rocks or martini style. If you want frozen:

Dump 1 small lime aid concentrate into a blender (if you have the time, thaw and strain it first). Fill up the lime aid container with tequila and add that. Fill the container half-full with triple sec (or other orange liquor) and add that. Fill 40 oz blender (normal size) with ice. Blend until it's a margarita.