Don't forget that you'll need a differences training sign-off for the controllable prop, so your instructor should take you through the subtleties of operation.
You will normally climb at full throttle and, when levelling off either maintain full throttle or throttle back to just below the desired cruise MP, then when you set the cruise RPM the slight gain in MP should bring you close to the desired setting.
Generally at about 8000' or above you will probably want full throttle and control power with RPM as you won't be able to achieve more than about 21-22" MP. All this assumes non-turbo.
High MP and low RPM is the most efficient and quietest, so long as you stay within the approved limits.