The easiest is to just generate a normal default map, with a biome layout you like, and copy over the biomes.png to your flat map.Or even use one from an RWG world.
Its just important that they have the same map size (8k for example).
The most "advanced" method would be to draw the map yourself.
Just open a biomes.png in a graphics program (photoshop, gimp), and draw the biomes as you like.
Copy the colors. Its important that they are full colors (no transitions).
As a last step, use the BiomeFixer tool in NitroGet (tools subfolder) to ensure a correct format.
The game is very picky about correct formats, so I have tools for the biome and splat maps.