Where are you located? Put this on your profile and people won't have to ask.
I've put in three of these, covered and uncovered. In both cases, the area was formed up as a part of the basement (continuous footings and poured walls).
So in your case, you'll want to over-excavate to allow for the footings. Form these up and tie into your existing footings somehow (re-rod dowels drilled in and attached to existing with epoxy, etc.).
Then you will also have to attach the concrete block wall or poured wall. You're going with C block walls, so you'll have to drill into what you have, attach bolts, then do same with what you're adding. If you're attaching to C block, then drill in the core of each block or however often as necessary, put in a bent piece of rebar, and leave a fair amount sticking out. Then go to the top of the wall and knock out a piece so you can fill the whole core solid with grouting material. If you knock out this piece on the exterior, maybe you can get a piece of re-rod down in this core as well.
If you are connecting to a poured concrete wall, then just drill holes, insert re-bar, and fill with epoxy.
When you lay up your c block wall for the entrance, you'll want to insert the re-rod into holes in the block, then bend the re-bar a bit. If possible, you can bend it before you lay the block. Maybe it is not even necessary to bend the re-bars, but it won't hurt. After you are done laying this wall, insert re-bar vertically down that core, and fill it with grout. You can fill as many cores with re-bar and grout as you like, I think minimum of every 4'.
If you are leaving the entrance open, you may as well cap it off with solid block or fill all the cores. It would look nice to have a rowlock of brick on top.