Make your simple syrup by combining 1 cup water with 1 cup sugar. Heat the mixture just until all sugar is melted (no granules remain) and liquid is cloudy-clear, Doing let it boil (you will lose some via evaporation and allow the mixture to contain more sugar).
Allow mixture to cool to room temp, then store in fridge is a sealed jar for up to 3 weeks. This should yield between 1 1/2 cups and 1 3/4 cups...so 22 TBS worth.
When ready to assemble coolers, first you want to supreme the limes. Like I described above. Once you have your limes cut into segments, place 2-3 segments in each glass and muddle them (with a muddler or wooden spoon). Get them all broken down, so you have pulp floating around your cocktail, yum!
Fill the glass with as much ice as you like, then vodka, then 1 1/2-3 TBS simple syrup (depends on the size glass you use and your preference for sweet).
Top with seltzer, garnish with a lime slice and maybe a few thin shards of lime skin in the glass and enjoy!!