David, I did commercial carpentry for quite a spell, so have a clue as to what you are after. The best tip I can give you is you need to decide and maybe even have on hand what you intend to use for a door, so you can get the block opening correct. This is called the "rough opening". One way, which is very labor intensive is to "tooth" the existing block by removing them carefully, cleaning all the old mortar off the ones left. They make block that has a finished end on it. You would want to use them, cutting a near full length and a near half length and remortaring them in lieu of ones you removed. As you put them in, you "slush" the holes in them w/ mortar, which gives you something to work w/ when you anchor either wood or metal frame. Good luck!!!