You just need a decent icon editing program, there are many available.

A popular (and very good) one is MicroAngelo, but that costs money. There are many free ones available, and it is possible to find high quality ones for free - such as my personal preference, IcoFX. You can find these two (and many more) by doing a web search for "Icon editor".


In IcoFx you can add a new icon to the file by clicking the "Insert New Image" button, then select the size/colors for the new icon (by default it copies the icon you've got selected at the time, automatically adjusting the colors/size as apt). Other decent icon editors offer similar functionality, just via a different button/menu.