I taught seminars on dyeing carpet from the mid 1980's until 1996.
That thing about the pad. Never heard of such a thing. And if it ever did happen, it would be due to a very poor job of application.
The idea is to spray enough for the fibers to be saturated, yet not over-wet the carpet. And it's not a difficult thing to do.
Cardboard around the edges...? No. It will get saturated and smear the walls. If you use a hand- held shield, a painter's plastic shield is the way to go. But there are liquid wall guard products that won't let the dye stick to the walls.
During the time I did carpet care, I used a HydroForce for part of it. I still prefer not to, but it can work with some care to not over pressurize or use too much heat.
I definitely recommend taking a course. It's not the kind of thing that I would suggest st to anyone that they read about and then try on a customer's carpet.