I think Overworld creation, in the design sense, aside from basics like "don't create perspective errors" and "avoid overly flat sections", would be something very hard to make a tutorial for.
You can boil level design down to a kind of "science", what works and what doesn't, the "Level design with a purpose" concept you see Nintendo applying to their platformers to this day in varying degrees; and we actually do have a good Tutorial for Level Design here in the forums
, which sounds like it's right up your alley, but creating an Overworld Map, to me, is closer to something like drawing a scene than it is a science of level design.
There's general advice for building one, like making sure the levels in your hack match what the Overworld looks like even down to details (e.g. if your Overworld has a hill and a river next to a level, try adding those elements to the background of them), try and make it varied but with transitions that make sense (A sea, that goes into a beach, proceeds to a grassland, going into the foot of a mountain, climbing said mountain and reaching the icy peak is an organic and natural progression, for a good example), try mixing your levels up with different times of day emphasised by Palettes and gradients in the BG, or different weather conditions like Wind or Rain to make the Overworld feel more alive and believable, play around with the graphics and Palette of your map, etc.. But that's all stuff you can opt to follow or not and you add them as you see fit. Nobody can really teach you how to draw your map.
Of course, for the more technical stuff, there are good tutorials included in Lunar Magic itself, like Veck said and there are a few good tutorials on Youtube, even if some are a bit outdated, they're actually good enough to at least get your familiar with how everything works.