You have the basics..the finesse comes depending upon the horse in question. On mine, I do the same in the trot to walk. I half halt to let the horse know ther eis something coming, sit deep, us my leg to keep him balanced and working under himself and then just hold with the reins, creating that invisible wall in fornt of him. As he gets more "tuned" just the sitting deep is enough to bring him down to the walk..I can also halt from there without adding any additional rein pressure.
From canter to walk, same thing. Half halt to let the horse know a change is coming. Sit deep as in really letting my weight drop down as opposed to sitting deep to sit the canter, stop following with my seat while applying leg pressure and just holding the rein position to create the same wall. We are doing mostly canter to walk transitions through the trot right now as he still has a tendency to fall on his forehand going down to the walk out of the canter. Haven't quite got that tweaked yet. :)