I want to propogate 'Photinia fraseri (red robin) to make a hedge.

In a book it tells me to take cuttings and put in a cold frame. I dont have
a cold frame and don't really have room for one. Could I put clear plastic
bags over flower pots? Would that do the job ok?

How long should the cuttings ideally be please, and how many leaves should
be left on at the top of the cutting? Anything else a complete novice
should know? Thanks for advice.