I'm working with the Mala front end right now. The design seems to boil down to creating a background image (which I attached a few posts back) and then placing elements the front end uses, such as game list, marquee image, screen shots, onto the background image. Then the creation of game lists, there are so many game rom images out there that to just present them all in one long list isn't practical. So I'm working on various lists to make it easier, I'm going to include a list of the games I consider a must of an "Arcade Legacy" then a list of fighting games, shooters, beat-em-ups, platformers, and maybe by manufacturer.
The tutorials on front end customization are okay, but what I've found more informative is downloading a front end layout, and then opening that layout in the designer to see what does what.
I haven't quite figured out switching between emulators yet.