Good luck with your humidifier which will help in cold dry air of less than 30% humidity especially if you and kids have sore throats and respiratory problems.
Blower or unblown depends. I think the unblown work fine but take a day or two longer to humidify depending upon how much your furnace runs but it shouldn't matter. Even though it doesn't blow a lot, a blown one will blow a chilled draft when the furnace isn't running that you may not like and I know I wouldn't. Either way, allow a week after install before you get the house at a higher humidity.
Water install. I like the others have seen too many saddle tee valves leak or not reseal for the summer or for repair. Put in a copper tee with a ball valve coming out and then either reduce down and solder or solder in an adaptor for a compression fitting. For double shutoff control, use a 1/2" solder by 1/4" compression straight stop after the ball valve. You may need to go to a parts house to get that particular size stop as home cheapo won't have it. Run your drain to where your a/c and water heater overflow run into such as the sump pump basin.
Electrical install. Buy a metal electrical box with a light switch and cover. Install in on the side of the furnace where you will run your wire out with screws. Attach the relay coming out of the top or use a double box to hold it all. Then you have a switch to shut it off during the summer, etc. Check the schematic and furnace wiring to tie into it. Only if you are lost in the wiring do you need one of those special wrap around sensors and if all else fails have an hvac serviceman do the wiring. Put your humidistat by the thermostat and run the wire to that point.
Sheet metal install. It should come with a template to put on the plenum to make your cut. Do not install it where the a/c coil is setting which is usually in the first foot and a half to two feet above the furnace on an updraft model. So stay above the a/c coil so you don't cut a hole in it and also the air flow isn't just right at the coil. If the return air runs next to it, then prefit your elbows, etc and set it up before marking and cutting. It may be a tight fit to make the corner on the elbow. Have a set of left hand and right hand aviation snips as that duct work is hard cutting.