How to create a multisize favicon using GIMP

For this example I installed GIMP, an open-source raster graphics editor, and created three icons sized 48x48, 32x32, and 16x16 px.

Do the same. And then take the following steps:

1. Open up GIMP and open, File > Open..., the 48x48 px icon:

2. Open the 32x32, and 16x16 px icons as layers, File > Open as Layers....
3. Arrange the layers by size, ascending top to bottom. Just drag and drop:

4. Finally export, File > Export As..., as favicon.ico:

5. Click Export:

Done!