If you’re new to using braided lines it seems kind of foreign and unnecessary to use a mono backing, but this is far from the case. Using mono for a backing on all your reels with braided line is vital! If you don’t and you set the hook on big fish the line on the spool will spin especially if you’ve put in some hours on the water with it. I’ve had it happen to me. I actually had a bulldawg get destroyed by a musky that hit it 3 times. Each time I set the hook the line on the spool spun while the drag had everything locked up tight! That was an expensive lesson! $20 for the bulldawg and a lost fish. Needless to say it was a lesson I’ll never forget!
The reason you have to put mono backing on the spool is because braid is coated with a waxy substance that makes it very slippery. Mono on the other hand stretches a bit and grips the spool very well. So all you have to do is tie the mono on the spool using an arbor knot or something similar and completely cover the spool with mono. There doesn’t have to be a couple hundred feet on it or anything just enough to cover the spool and provide traction for the braid. The next thing you need to do it join the lines. I prefer to use a double uni knot it’s easy to tie and you dont have to worry about the lines being different diameters. ANother option is the blood knot which works best if you have line that is the same diameter. After you have joined the 2 lines make sure the tag ends are clipped very short. Now spool up the reel with as much braid as you like and you’re done.
Braid is pretty expensive so a lot of people fill up half the reel with mono and then put 75 to hundred yards of braid on top of that. In most cases you’ll never even see the mono backing. The great part of doing it like this is that you can use what’s left of the braid on another reel. A great way to get more bang for your buck.