Please correct me if I'm wrong, Nikel, but I believe the first part of that explanation would better read as follows (italics added):
Procedure: To form a Corps, first select an unattached general and press the 'Enable Corps Command' button. Alternatively, remove an attached general from his current stack so that he is represented by his own unit tab and then press the 'Enable Corps Command' button. This forms the Corps. Next select the Units and any other generals to be assigned to the Corps, transfer them to the new leader's tab, and click the 'Combine' Special Order. The Corps is then combat ready.
I'm not trying to be narky here, but in many places the manual is not up to the mark and serves to confuse rather than explain.