Optional Shader Configuration - Covarr/covarr-bezel-bezels GitHub Wiki
Directions
From the Quick Menu, scroll down and select Shaders.
Select Load Preset.
Open shaders_glsl and then handheld.
If you are using Nintendo_GB_DMG_Shader.cfg for your overlay, select gameboy.glslp. See Gambatte‐Specific Configuration for more information.
Otherwise, select the shader of your choice. Some good options for emulating handheld systems that won't eat too much performance include lcd3x.glslp and simpletex_lcd_720.glslp.
Select Save Preset, followed by Save Content Directory Preset.
Notes
Unless otherwise noted, my personal recommendation is to use the lcd3x shader alongside these overlays. That's absolutely just down to taste and device performance, though; use what you prefer. simpletex_lcd_720p is also a great choice.
For Game Boy, I strongly recommend using Gambatte. Nintendo_GB_Colorization.cfg in particular is specifically designed for it.