I agree with Jay. Been there, done that, and almost screwed up. I installed a different brand but I think it is similar. It came with a template to stick on the plenum. I had half of the hole cut with a reciprocating saw before I realized and remembered the air conditioner condenser was sitting on the furnace right behind the hole. Luckily I didn't hit it with the saw. I taped the cut and moved up a couple feet. Mine was the type that had a damper and a 6" pipe that went over to the return as a bypass.
If it comes with one of those funky and eventually leaky saddle valves to tap into your plumbing, I would trash it, install a regular tee in the plumbing and then add a ball valve to shut off the water to the humidifier when it isn't being used. After the ball valve, use a reducer to the 1/4" tubing or just put an 1/2"by1/4" angle stop on the end to make your reduction and also give double shut off protection. You may have to go to a plumbing supply store (or home cheapo, etc.) to find one that goes to 1/4".
On your wire from the humidifier to the inside of the furnace, I would install a metal work box on the side of the furnace with a regular on/off switch on it. That way you can cut the power to the humidifier if you want to work on it (even if it is only 24 volts). Good luck