set things in motion, put a plan in motion
If you 'set something in motion' it means to take the first step in starting something working or moving. If you set or put something in motion you trigger or influence a series of events. This idiom comes from the expression 'set the wheels in motion' which suggests beginning to move things forward with a project or plan. You were here this used often in the expression 'let's set this plan in motion' where it is used to authorise or encourage the beginning of something. This expression is probably more likely to be used in professional contexts.