Add the raspberries and sugar to your container. Gently mix them together so that the berries are fully coated in sugar.
Seal up the container and let it sit at room temperature for around 30 minutes.
Check on the mixture, and give it a stir. At this point you will probably still see sugar granules.
Seal up the container again and let it sit at room temperature. Check it after 30 minutes and stir again. Continue doing this until you don't see any more undissolved sugar granules.
Once all the sugar has dissolved into the raspberry juices, give it a final stir. You can use your spoon or spatula to break up any chunks of raspberry you still see in the mixture.
Add in the water and stir.
Set up a funnel to go into your storage bottle, and put a strainer on top of that. Pour the raspberry mixture through the strainer so the liquid will go into the bottle.
Use your spoon or spatula to squeeze out any liquid left over in the raspberry pulp that's in the strainer.
Seal up your storage bottle. You can use the syrup just after making it, or keep it in the fridge for up to three weeks.
Notes
If you don't have any fresh raspberries available, you can make this recipe with frozen raspberries.
You can make this syrup in the fridge instead of at room temperature. It will just take longer for the sugar to dissolve in the raspberry juices.