Like everyone else said they need an experienced rider, but they should always be looser than your snaffle rein and a soft hand
Like Hayley, I only use them in extreme cases, like when we are bringing a hot horse back to work after several months off, we use them, but only as emergency breaks, usually leaving loops in them unless if we need them. Again reinforcing what Hayley said, we only have experienced riders using them who will only use them when they have too, and not accidentally.
In the hunter world, a lot of people use them in their warm ups to get their horse going round, but I don't like this because the frames end up being artificial and they usually aren't actually engaging their hind end, and usually soft horses end up with wayy to much pressure in their mouths.
In terms of how you use them, they sit over the neck, go through the bit on their respective side from outside to inside, and clip on the bit. You hold them between your middle and ring finger and your normal reins between you ring and pinky fingers. Make sure that before you put them on your horse that you have a reason to and your trainer sees them fit.
I only use them in extreme cases such as extreme pulling, or an extremely locked jaw, or if the horse is tossing it's head really bad. You need to be a very experienced rider, and know when to take and when to give. Theyre a wonderful tool, but you can really mess a horse up if you aren't careful.