The basic rule for making sugar syrup is to use equal parts sugar and water; so for every cup of water add a cup of sugar. Start with that ratio and, if you want it more or less sweet, adjust the next time you make it. Bring the water to the boil, then dissolve the sugar in the water, stirring constantly.
Once dissolved remove from the heat. The longer the water boils, the thicker the syrup will be – and you don't want toffee. You're after a thickened but still liquid texture and remember it will thicken further as it cools.
Once cooled completely, store in a screw-top jar. Your syrup will keep in the fridge for several months, but if you need it to last longer, add a splash of vodka.
For a fuss-free natural alternative try agave syrup.