Not sure about guide - probably somewhere, anyway:

To put island on map you can just edit map_heights.tga (you definitely need to have deleted / re-named map_heights.hgt in campaign folder and base)
Just add area you want in a greyscale colour RGB 1,1,1 or lighter. You can play with heights to get hills later.

Delete map.rwm and launch game - that will create a corrected_regions.tga for you in RTW folder (with .exe's) that will have just assigned the island to what it thinks is nearest region, if you copy that and re-name it as your map_regions file you can change the ownership of the island to whichever region you want - or use it for new region when you alter the text files.

Re: getting heights and regions maps to align, its not quite as simple as 242 = 121 because of the 2x +1 pixel difference (though obviously quite close) If you have Photoshop or Photoshop Elements and resize a copy of the map_heights to the map_regions size using 'nearest neighbor' resizing it will show you the correct coast shape the game uses (PaintShopPro resizing distorts it slightly)

You haven't done anything to map_ground_types yet so island will just appear as default fertile ground type in game. To set that use layers / selection process in your image editing program to get area same size as in map heights and add the ground types you want.