Zip ties and electric tape or thin strips of gorillas tape.
First I attach the muzzle with the 4 straps that comes with it. Then I wrap the side hardware with thin strips of a white flannel wraps (or strip of an old polo) and just tack it down with glue and wrap a piece of electric tape around the ends. It lasts for a season (only necessary if your horses gets rubs).
Then once you know it fits right you can add the extra stuff. I would let them wear it a few days because new halters and those straps stretch, also if you horse is new to muzzles maybe just use it that way for a few weeks or until they figure it out. Once you know it fits I would wrap the straps with electric tape to ensure straps don’t get loose.
But if you have to add the extra “armour”, hang the muzzle up so it’s nice and level and add zip ties half way between each strap. I make sure the locking nub is on the outside of the muzzle, not inside! And not up on the halter. They will tear a hole in your horses face overnight so you want them stable and unable to move. Slowly close the ties and make sure they are the right length when you pinch them closed because you are going to wraps them with tape tightly so that nub can’t move (strips of gorillas tape lasts longest). Then after that is done I took zip ties and created crossbars on the two back corners and wrapped that. That might be overkill but since my guy already learned how to get grass out of those corners, so necessary!
The other thing I find useful is that pad on the halter chin. It’s from thin line. They sell it to go with the flexible filly muzzle. I bought a pair and use one on the nose to go with the FF muzzle, and the other on the chin for the Fjord. Since he dedicated his summers to trying to evade or rub off the muzzle, every bit needs to be padded or fleeced.