With Jeans: Wear it over a fitted tee with straight or bootcut jeans and ankle boots. Keeping the top underneath closer to the body balances the longer cardigan.
With Trousers: Layer it over a simple knit with straight-leg trousers and flats. Leave the cardigan open so you keep that long line through the front.
Over a Dress: Add it over a simple midi dress with boots or flats. The longer length gives you coverage while still showing the shape of the dress.