Get a heavy bag & hit it alot!
I was told you should kick the bag 100 times a day, on each leg to condition your shins & it worked. Also improved my cardio a great deal.
I've never focused on my elbows or knees, but it's the same, just use them. . . alot!
Don't roll them, or hit them & don't go kicking hard objects!!!
It's a gradual process, you can't rush it, if you do you will only damage yourself.
Sorry no magic formula, just patience & hard work.