Get a pick like the one in the image. Clean the inside edges of the grouphead.

Clean all surfaces. First, focus specifically on the top rim that comes in contact with the portafilter. This helps the portafilter to seat correctly.

Focus on the bottom surface. Cleaning this surface helps the gasket to seat correctly.

Use compressed air to remove all excess debris from the grouphead.

If you have Version 3 do not install the brass disk until you are finished installing the new gasket.