Don,
I have my own way of doing this, but it may not be the best way. Take a small screwdriver or sharpened nail and depress that top of the spring end into that mag body detent, while sliding the bottom plate over the depressed spring. Some bottom plates have set so long that they have become rusted and they may be a little hard to slide off by hand pressure. If that is the case I use a padded large screw driver as a lever, between the bottom plate and the mag. Be careful not to apply to much pressure and bend anything. Soak in light oil, if required. I just use the larger screwdriver to get the bottom plate to start moving and determine who much rust or grit has been built up. Once you slide off the bottom plate, that spring may have a mind of its own. It will shoot somewhere, if you are not careful. Be sure and take the whole mag apart to inspect and clean. Use the opportunity to get the mag interior, base plate, spring and follower as clean and well oiled as possible. If there is rust or grit on the inside of the mag body or bottom of the follower, get it out. Getting it back together in 5 to 6 seconds, I have learned a trick. Take a pair of medical forceps to grab and depress the spring as you guide the wire tip end back into the grove or nitch. The forceps are the perfect tool as they allow you to manipulate the spring back into a compression mode exactly where you want it as you slide the bottom plate back into its closed rearward position. After the bottom plate passes the nitch, then release and withdraw the forceps and complete the slide in reverse. Warbird is the true pro at everything HSc related, so he may have a far better procedure and I would always yield to his expert counsel. That is the way I do mine. Good luck with yours.
JIM