If you're using frozen berries, allow them to thaw first. I actually microwaved mine on defrost. 

Purée the raspberries using a blender or potato masher.

Strain the raspberries to remove most, if not all, of the seeds. I just used a simple mesh strainer.

Mix together the water, lemon juice, sugar, and puréed raspberries in a small saucepan.

Before heating, whisk the corn starch into the mixture. Whisking will ensure a lump-free sauce.

Bring to a boil on low to medium heat and simmer for about 3 minutes, or until your sauce has the desired thickness.

You can pour or spoon this delicious raspberry sauce on pancakes, waffles, ice cream, etc. I actually swirled most of mine into my cheesecake batter.