I make them all the time in summer people tell me they are good.
You need.
Lime cordial and fresh limes
Brown sugar (Demerera is what I use)
White rum
Soda water
Crushed ice
Fresh mint leaves.
So in a jug put say 1 cup of white rum
put around 3 tablespoons of the sugar in the cup and dissolve with hot water (use as little water as you can) then add to the jug
Put around a 1/4 cup of lime cordial in the jug (you'd think fresh limes would be better but people seem to prefer the cordial)
Mash the mint leaves up in your hands, you don't want to shred them just help the goodness out, and add to the jug. Mix all that up.
Then add lots of crushed ice and finally top up with around 3 cups soda.
Serve and garnish with a qtr lime and a mint leaf.