If you are using raspberries and wish to strain the seeds from the raspberries you can do it now by mashing the raspberries through a fine sieve. (You can do it at the end if you prefer, but I think it's a little easier to do it before it is cooked.)
If you are using cherries, chop finely in a blender of food processor. Hand blender works ok.
Stir together fruit and sugar in saucepan.
Mix water and cornstarch in a small bowl and stir till smooth then add to fruit in pan.
Cook over medium/low heat, stirring often, til the fruit have cooked down and the sauce is thickened. This will take 7-10 minutes.
You can sieve out the raspberry seeds at this point if you haven't already done so. It's also fine to leave the seeds in. Of course, the sauce has a very different consistency with and without seeds.