Place the sugar, mint, and one lime wedge in the bottom of a sturdy glass, and muddle to release the oils from the mint and juice from the lime
Add two more wedges and gently press to release more lime juice, mixing to incorporate the existing juice and sugar
Add the rum and top with ice and soda water
Taste and add more sugar if needed
Notes
If you have simple syrup, you can use that instead of the sugar. The sugar does help release the oils from the mint though, acting a bit like sandpaper. You can add more sugar as needed depending on how sour you like it.