Here's my process using the free and open source image editor, GIMP (I'm using version 2.10)
- Launch GIMP
- Open your R/G/B/IR images:
File > Open as layers/CTRL-ALT-O - Optional: Add tags to your image layers:
Right click layer > Color Tag > Choose R/G/B/Gray (IR) - Set mode to grayscale:
Image > Mode > Grayscale - Align layers:
- Disable layer visibility (press the
eye) for each layer except the bottom one - For the next layer, make visible and, making sure the layer is selected, set
mode > subtract.
- Disable layer visibility (press the
- Select the move tool and use your arrow keys to make sure the layer is aligned with the layer below.