Create a folder in your %APPDATA%\SpaceEngineers\Mods folder.

Create a .dds image file for your icon. (E.g. using paint.net with save as BC7 sRGB, DX 11 +). Make sure to remove any unwanted background, etc.
Save it in your newly created mod folder.

Create a LCDTextures.sbc or copy the one from the example to your mod folder into the folder "data".
Update the SubtypeId (should be unique or it might get overwritten) and TexturePath (the relative folder path where you saved your .dds file). Same for the TransparentMaterial, I suggest using the file from the example mod and to adapt the SubtypeId and Texture to point at your icon.



Create a MapMarkers_Config.xml or copy the one from the example to your mod folder into the folder "data".
Update the Displayname (it's what will be in the selector dropdown) and LCDTextureDefinitionSubtypeId (you set this up in the previous step).

Add the newly created mod to your game and test if the map marker icon shows up in the terminal.
If not, feel free to contact me on discord so I can help troubleshoot.

I like to use MDI icons [pictogrammers.com] since they are free and have lot's of options.

Select one, select "Advanced PNG Export".
Select white as the Foreground Color, keep the Background Color as is.
Change the Size to 256px and then click Download PNG.

With this, you can continue and save it as a .dds (e.g. using Paint.net)