How do I convert a .PNG into a .ICO?

You can use the opensource imagemagick convert utility, that can take several images and pack them in one single icon file. It can also do re-sizing and lots of image manipulation. Imagemagick contains other tools for image handling, is available on multiple platforms (Linux, Windows, Mac Os X, iOS) and can be batched easily, for example in Continuous Integration pipelines.

Here is a basic example exporting a svg file to a png file:

convert icon.svg -scale 32 tmp/32.png

And how to pack several several of such png files in a final ico file:

convert tmp/16.png tmp/32.png tmp/48.png tmp/128.png tmp/256.png  icon.ico

Note that the standalone convert binary may not installed in the default installation. On Windows, you must check the (quite explicit) Install legacy utilities (e.g. convert) option. In case you missed it, simply use the slightly longer magick convert command rather than convert alone.

For one-off tasks I usually just cheat: ConvertICO.com .

If you will be doing this fairly often, you may want to consider the free Photoshop plugin.

Install ffmpeg from: Windows: http://ffmpeg.zeranoe.com/builds/ OSX: http://ffmpegmac.net/

From the shell, use the following command to convert PNG to ICO.

ffmpeg -i img.png img.ico

Also if you use ffmpeg regularly, don't forget to add it your PATH variable.