I know most pots are round but when root pruning think of a square. Get your plant out of the pot. Cut halfway between the top and bottom then cut off the sides to make a square. Fill your pot with enough fresh soil (my mix is 3 parts soil 1 part manure) so that when you put your plant in it is near the top of the pot, then fill it in with your mix. I have a small saw I use just for pruning.
Thanks for the instructions. Sounds like a good thing to do.