Are you asking how you actually to disable the button? Or do you want to know how to create a disabled (greyed-out) version of the image that appears on the button?
If the latter, then the answer depends on which image editing tool you use. Most of them have an option called something like "grey out" or "disable". Or, failing that, you can usually change the colour depth to "grey scale".
If you would care to let us know what icon editor you are using, I - or someone else - can give you specific instructions.