Materials:
Greenhouse
1.5" PVC Piping
PVC T Connector
PVC 2 to 1.5' Reducing Coupling
Reli-On Ultrasonic Humidifier
Trash Bag
Timer
Duct Tape
Hacksaw
Start out by assembling the greenhouse. They go together quite simply like an erector set. Instructions will be included.
Now you need to lay a garbage bag down on the floor, especially if you have carpet.

Here are some pictures of the materials that you should have gathered.
Reli-On humidifier with adjustable humidity output.

The adjustable humidity output.

T Connector

2" to 1.5" Reducing coupling

If your are choosing to use the products listed, then the humidifier has a hole on the top that is approx. 2 inches across for humidity output.

This is the perfect fit for the reducing coupling.

Using duct tape, make a really good seal, attatching the coupling to the humidifier.

You will now need to cut the PVC using the hacksaw. In this situation, using 2 4-tier greenhouses, there will need to be a 43.25" peice and 2 30" pieces of PVC. These 30" pieces of PVC may need to be longer, depending on how far away you place the greenhouse from the humidifier. If you make it like I do in this thread, then 30" peices will work fine. Also, if you are only using 1 4 tier greenhouse, then the second 30" piece is not needed.
Attach the T connector to the longer peice of PVC.

Fill the humidifier with water at this point.
ONLY USE DISTILLED OR SPING BOTTLED WATER.
Doing this will greatly increase the life of your humidifier.
Place this peice of PVC into the humidifier with the T aiming towards each greenhouse.

Now we shall work with the 30" peices of PVC. Using a pen, mark off 4 inches from one end of the PVC, and 2 inches from the other. Inbetween these marks is where we will need to drill holes for the humidity to come through. Again here, if you do not use the same humidifier/greenhouse spacing that I use, your numbers may be different.

Now you should seal off the end you marked 2" down with duct tape.

Cut a small hole in the greenhouse plastic where the PVC will go through. Make the hole small enough that there will be a tight fit when you put the PVC in.

Now place the PVC into the hole, and connect it to the T connector.

Do the same with the other side. If you are only using 1 greenhouse, then you can just seal off the other side of the T connector with duct tape.

You should also place duct tape around the opening in the greenhouse to prevent any humidity loss.

Now you will need to setup the timer. I am using a digital model avaliable at Walmart for $15. You will probably need to setup the timer to run 3-5 minutes every 2-4 hours. Also, dont forget there is a humidity output on the humidifier that can help with getting the proper humidity adjustment.

This is by far not the only way to accomplish this goal, hopefully just some inspiration. Be creative.