Ratios:
1 lime juice
1 simple syrup
1½ Gin
Lime juice must be from actual limes, not "RealLime"® or Roze's® Lime juice. Squeeze some damn limes! You can freeze lime juice for later use and it works fine.
I base everything on however much juice comes out of a lime, by weight. It's easy to use a scale for this (but somewhat nerdy)
combine them all in a shaker with a bunch of ice, and shake the heck out of it. This chills the mix and thins it with some water (this is important)
Alternatively, if you want to pre-mix it and just keep it cold (no shaker, no ice):
Ratios:
1 lime juice
1 simple syrup
1 water
1½ gin
this assumes you do it in advance and keep it *cold* in the fridge or freezer, adding ice only when it goes into a glass. If you add ice to the warm mix it'll get watery really fast as the ice melts.