For a very easy to follow method of taping a Corgi's ears (plus illustration) go to:
www.phi-vestavia.com/EARTAPING
14-16 weeks is about the time to tape if a Corgi's ears are not properly erect.
If you have a Corgi who doesn't fit the medium (ie small, large), don't go by weight, go by sight and feeling (easily feeling the ribs). Don't 'spoon' in the food just because you have some notion your Corgi should be a certain weight. Overweight as per the size of a Corgi, is a killer.