Enough water to just cover them
~1dl lime juice
Add raspberries and water to a large pot and bring to a slow boil. Once the water is almost boiling (medium-sized bubbles, not violent bubbles - or when the water is 90C if you have a meat themometer), mash the berries up against the side of the pot.
Cover a colander with a dishcloth that you don't mind staining purple and pour the raspberries through the dishcloth into another pot. Leave to drain for awhile until it's stopped dripping, even when you move the mess around a bit (I had to do this in batches, as my colander could only fit about half at a time.). Discard the contents of the dishcloth.
Bring the lemonade to a boil again, adding sugar and the lime juice. Once it's almost boiling, taste and add more sugar or lime juice as desired. Transfer to bottles and put in the fridge to cool.
I found that this ended up being quite concentrated, so mix it up with water / sparkling water before drinking it, but it all depends on how much water you added when mashing the raspberries in the first step.
Yummy! Lots of work, but worth it! This would taste really awesome mixed in a drink as well.