idontbeleiveit, there's a how to over on the blog, in the Pictish Cavalry post. Short version is make up the design using PhotoShop and InkScape, from suitable graphics, then print onto decal paper, clear-coat and away you go. Also great for measuring sticks and Saga dice!
Cyrus, the short answer is of course, because I can! More seriously, three points of hearthguard is 12 figures, and can be got on the table a lot quicker (literally half the time) of three points of warriors (24 figures). And let's not think about three points of levy!
From a strictly archaeological perspective, any warlord who could run a warband with that many hearthguard must have been powerful beyond comparison, lucky beyond belief and rich beyond the dreams of averice, so I agree it's not a very historical lineup, but my reading of Saga is that that doesn't matter too much.
As for whether it's a good idea in game terms, well, time will tell, but with 12 hearthguard around him, my warlord is gonna take some killing!