The video here is probably the most comprehensive instruction set you can follow:

Here's a quick summary for the weldless kits:

1.Drill a clean 9/16” diameter hole in the sidewall of your metal vessel, preferably with a step or “uni” bit. The center point of the hole should be 5/8” higher than the bottom of the vessel or any curvature in the sidewall.

2.Unscrew the threaded bushing from the elbow fitting.

3.Leave the metal washer(s) on the bushing and insert it through the hole in the vessel from the inside.

4.Push the clear silicone washer over the threads on the outside of the vessel.

5.Hold the tee fitting (or elbow) and tube in front of the threads and turn the bushing from the inside to thread it in. The silicone washer should crush slightly but do NOT over tighten. The image to the right shows a properly installed weldless elbow kit. Note that the silicone gasket is the only thing between the fitting and the outside wall of your vessel. ---->

6.Using a couple wraps of teflon tape on the threads, thread the thermometer into the front of the tee until it starts to get snug when the top of the thermo is centered to the sight tube. (this only applies to the thermo combo kits)

7. Make the tube straight up and down (plumb) and mark a spot to drill the 1/4" hole at the very top of the vessel for the eyebolt.

8. Level the vessel, apply a vertical piece of masking tape to the side of the tube, and add a carefully measure gallon of water at a time noting each gallon on the tape. Empty the vessel and remove the sight glass. Cut the number strip into individual numbers, remove the rigid paper off the number, apply at the correct level on the tube. Remove the vinyl backing. (It's probably better to view the video to get this).

IMPORTANT Warning about potential heat damage in extreme burner situations....

 The polycarbonate tubing is rated to withstand boiling temperatures and a bit more. In some situations, the heat from your burner will curve around the sides of your heating vessel and overheat the tee fitting. It is advised that you do a water test with full burner flame and check the temp just under the fitting with a handheld thermometer. If you read over 250F, you should fashion a simple heat shield. Tests have shown a simple piece of aluminum roof flashing can reduce the fitting temperature by over 100 degrees. This will prevent boil bubbles in the tube as well as keep the tube from melting.