I did do a load of screenshots but Ofnuts describes the procedure anyway.
I start from the bottom - the grey background and work up-over. The drop shadow is default Gimp. There is a brighter spot just under the flipped F - I used the dodge tool for that 50 % opacity fuzzy brush, one click. FWIW the Gimp 3 xcf attached.
I start from the bottom - the grey background and work up-over. The drop shadow is default Gimp. There is a brighter spot just under the flipped F - I used the dodge tool for that 50 % opacity fuzzy brush, one click. FWIW the Gimp 3 xcf attached.