The first thing you have to do is select an army stack. I'm not sure if it can be turned on an off easily anymore, but you should see the 'army wrappers' on the left side of the map, one for each army you currently have.
If you click on one, the map will focus on that army stack--and select it I believe--, if not click on the stack on the map to select that army stack. The army stack will now appear in the Stack Panel--the section at the bottom-middle of the map, where you see the units in the selected stack.
Now press <shift> to see the command radius of the army--it will appear as a blue overlay surrounding the army stack for a number of regions in all directions. The higher the strategic rating of your army commander, the larger the radius.
The ** or *** leader you wish to give corps command must be inside this radius. If he is not, you must first move him to inside the radius--or visa-versa--or select another army stack which has the **/*** leader within its radius.
Once you have assured that the **/*** leader is within the radius of the army to which you wish to give command over the new corps you wish to create, select the **/*** leader, select the organizational special orders group button (with the tent on it) just above the Stack Panel, and below the Stack Tabs, to display all the organizational special orders buttons to the right of the SO group buttons.
The second from the left, with a leader portrait on it, is the Create Corps SO button. Simply click on it and the selected leader will gain a brand new corps command. That's it!
