A garland on a banister is either the best thing in the house or the most obviously store-bought, and the difference is fixing and continuity.
Length first: measure along the handrail, not the floor, and add a third for the swag. Garland that's cut to length hangs flat and looks exactly like what it is. The swag is the point — consistent drops between fixings, deeper than feels correct, because it reads shallower once you're standing below it.
Fix at the newel post and at consistent intervals with a method that doesn't mark the rail — floral wire over felt, or fixings sized to the baluster. Lights go on before the greenery is dressed, batteries or a plug run planned so the cord doesn't cross the tread. And whatever happens, nothing may narrow the walking width of the stair or sit where a hand needs the rail. That's a safety line, not a styling preference.