I'm using a sprites sheet on my 2D game that contains different buttons.
I would like to create other buttons with custom icons because I'm adding new features to the game that need a specific icon.
The sprite sheet is in png.
Screenshot shows part of the sheet with a list of buttons (Great work by the author) where I simply added two rows of clean buttons where I would like to put some custom Icon.
In this example I'm working on a bell which I downloaded in white png format, added as a layer, filled with a gradient for the first green button on the left.
What I need now is to create the inner shadow effect to create the cool depth effect like in the other buttons.
Do you have any technique to suggest in GIMP or other free drawing tool to create such effect?
Thank you in advance.
P.s. Buttons are small in the game so I don't expect to have the fantastic details of the original buttons; good enough would be ok.
EDIT: Thank you @BillyKerr, with your detailed instruction I made it.