I just did this myself, and I used a 3/32" punch. The trick is to have a punch long enough to flip the inner magazine floor piece vertical.
I put Pierce +1 mag extensions on both of my factory G26 mags, and one factory baseplate was a complete bear to get off, and the other factory baseplate came off "relatively" easily.
I followed this video:
http://www.glocktalk.com/forums/show...61&postcount=7