Well, you get your pot, and fill it with about 2 parts water to 1 part sugar (though I like quite a watery syrup - if you want a thicker, goopier one, just use more sugar, less water), and a good fistful of raspberries *queen of precise measurements*. Bring it to the boil, and then let it simmer until it's reduced by about half (again, leave it for longer or shorter depending on how thick you want it). Let it steep 'til it's at room temperature, then strain off into your bottle, and smush the raspberries around to get any extra juice out.