Deploying an SMB is not a terribly difficult skill, but it does take some practice to be able to do it smoothly and safely. I've never used one on a reel, but always on a finger spool. In my experience, reels are bulkier and more likely to come loose or undone.
I would recommend learning the skill in fairly shallow water but close to the bottom, so you have a good immediate visual reference for your buoyancy. You can carry the spool pre-rigged on the bag, so you don't have to do any threading underwater. You use a double-ender to secure the line to the spool so it doesn't unravel while you are diving. Then clip the double-ender off to your gear somewhere when you are ready to shoot the bag. Adjust your buoyancy, because when you fill the bag, it will want to drag you upwards. (I orally inflate mine, so adjusting my buoyancy to neutral with full lungs is perfect.) Inflate the bag, and check above you to make sure you're clear. Pull the bag out until the knot has cleared the spool, and then let it go. You'll have to take a deep breath when you let it go to avoid sinking! Then readjust your buoyancy to neutral.
Doing this in midwater (at a safety stop) is much more difficult, because you don't have good visual references to maintain your buoyancy, which is why I recommend practicing it at first near the bottom.
