I don't know how you'd fix the corner, but I'm pretty sure I know what you did wrong.
When a pattern tells you to increase across a row it means to space your increases evenly along that row. So, in your case you would want to do something like K5, (M1, K11) 9 times, M1, K6.
I use this
site to help me out with calculations like that. Good luck!
ETA: of course
, while I was writing, Ingrid beat me to it! Oh well, it's a good link to have if nothing else!