I've been meaning to write a guide to creating Bezels for some time, but have put it off. They're not difficult to do at all - it's just that the information that's out there isn't written for idiots like me. Once I'd figured it out I wondered why I found it difficult in the first place.
Still - if there is interest in a (lengthy) explanation, than I'd be willing to give it a go. It's probably not worth the effort for just one person to read, but if there's more I know I sure would have appreciated an idiots guide when I started.
On another note - Mame is founded on sharing. As such, there is nothing sacrosanct about my LAY files. If I recall correctly, Mr. Do actually pointed out an error in my LAY files - but since everything works I've never changed them. Point being, if my LAY files help you, then use what works.
Having said that - LAY files alone aren't the complete solution - it's also about the template you use to complete the artwork.
The rules for me, from the beginning, was that I wanted to maximize the gameplay area - since first and foremost, I want to play the game, not look at pretty pictures. There have been times when I've really really wanted to increase the Bezel size so I had more room to create a bezel - but I resisted because on standard monitors the gameplay area quickly gets ridiculously small. IMO, of course.
For me it helped to have a "philosophy" or goal in mind when I started. Mame users want authentic artwork first. If authentic artwork exists, I see no reason to mess with it (although I have some games where I feel the Bezel is too big, and I might one day go back and make them fit my own templates to maximize game screen size.)
So, I target games with no official artwork. Better something than nothing, sort of thing. I'm not saying anyone should follow my example, just saying it helped me.
Mr. Do has an automatic LAY file creator on his web site (although I'll be honest and say I found it too complicated to use). So I figured out how they work and now use Notepad. Still, maybe Mr. Do's tool will work for you.