I once spent a bit of effort perfecting a compilation of my own garmin maps using mkgmap. Then lost it. And even if I'd found it would probably have forgotten how to run it.
So in the expectation that I will do the same again, I created a makefile for it, and put it on line on github - posting here just to share really in case it's useful for others.
I have copied and modified the default style to remove all points (shops, etc - I don't need them showing up - makes town centres look too cluttered) - pointless lines (footpaths, contours, political boundaries, constituency boundaries, hedges, pilons, ley lines, ... etc etc), so I've basically got just roads and areas (forest, lakes, etc) - looks nice and clean and uncluttered. Hence having named the style 'clean'.
https://github.com/bjtaylor1/garminmapsI've made it so that you can run 'make' again to update your gmapsupp.img and it will run osmupdate to (I believe) download patches and apply them, rather than redownloading the whole file (which could be large) again.
Also you can have just the country (or countries) you want in it. For instance I just have britain and france.