The Usage Guide needs some updating following changes to the user interface in Role Scoper 1.0.0 Beta (where the answer to your question becomes much simpler).

If you are using version 0.9.27, the Usage Guide should suffice.  So in this transition period, I'm not sure which instructions to give you.  After a bit more testing, 1.0.0 will be a much easier solution.  Are you game to try it out?  If so, the main clue is:

• Restrictions > Pages 
• set a default Restriction on the Page Editor role

• Roles > Pages
• assign Page Editor role to desired Groups, for desired pages (and their subpages, if desired)