Add water and sugar to sauce pot and turn on heat to medium-low.
Stir so that water and sugar are mixed a bit, then let come to a simmmer. If simmer turns to a rolling boil, drop the heat to low immediately. Keep a damp pastry brush or damp paper towel handy to wipe the sides of the pot to remove any sugar above the water line.
Let simmer until sugar has dissolved. Then take off the heat and set aside for 5 minutes to cool a bit.
After it has cooled a little, use a funnel to carefully pour into your swing top bottle (or other heat-safe container). Your contents will still be on the hotter side, so take your time during this step!
Cool bottle on counter for an hour before storing in the fridge (best enjoyed within 3 weeks).