I am having trouble figuring out the path to the folder where I have to put my custom icon png files.

On the wiki (https://puffish.net/skillsmod/docs/creators/configuration/icons#texture-icon), it says "The following example uses texture located in textures/icons from a data pack named example.". I tried that path, and all the variations I could think of and can't get it to work.

Could you specify the whole datapack path to make it easier?

The documentation is wrong here. You actually need to put that custom icon in a resource pack. This is because data packs can't add assets.

If you want to keep both data and assets in one place then consider making a code less mod which can work as a resource pack and a data pack. You can use a new Template Generator to easily create all files needed to make such code less mod.

I tried putting it on a resourcepack as well.

But tried again now and it worked. Maybe I forgot to reload the textures when I tried it the first time.

Anyways, its fixed, thanks for the help!