This tutorial is nothing more than information shared on PIBB a few years back. This effect was worked out in PI by either Cedge or Howie from a Photoshop tutorial.

Basically, infrared is reflected heat from photographed objects, not an inverted effect or negative effect.

Go here for a really comprehensive combination of articles on infrared photography and effects.

That said, let's begin.

Open an image of your choice, preferably one with a good landscape. CTL D to duplicate it and close the original. Resize, if necessary.

This first step is optional, but it does affect some photos. Go to Photo, Enhance, Diffuse Glow:

Use these settings:

Adjust, Calculation:

When the window opens, use these settings:

Go to Edit, Copy and then Edit, Paste, As an object. Alternately, you can use Selection, All and Selection, Convert to object.

Next, use Photo, Blur, Gaussian blur.

When the window opens, use the 5th or 6th thumbnail, depending on your image.

R/C and choose properties. In the pop-up window use this setting:

You might also want to play with the transparency, again depending on your image.

If needed go to Photo, Light, Brightness and Contrast:

and click on the bottom right thumbnail once or twice.

When you are satisfied with your image, R/C and Merge All.

Here are some samples I did with this effect:

I also made a Task which you can download here.