my two cents....it can be easier said than done...Its all about pitch, trim, power....this is what i taught, single engine aircraft C172, i had the student trim for level flight at 90 KIAS which was about 1900 rpm...Then have the student reduce power to 1700 rpm. the same practice works for multi engine normal ops and single engine work, except you change MP.

practice single engine ops in the practice area first, once your zero thrust is set, and youve trimmed for zero sideslip, once trimmed for level flight you can reduce power and retrim for a constant rate descent

practice making power changes in terms of 1" MP to 2" MP (changes on your operating engine. Set the change and see what happens.