Q.what do I need to do to over winter a montbretia?
How do I winter over this plant?

If you are in zone 5 or higher (learn your zone here: https://www.gardeningknowhow.com/planting-zones/usda-planting-zone-map.htm )the plant should be fine in winter. Hopefully you have a variety that is hardy in your area. If you wish to keep it in the pot, you could dig a hole and sink the plant and pot into it; that way, the earth will insulate the roots from killing cold. Cover the whole thing with mulch, as you would for plants in a bed. This article has more information that you should find useful: https://www.gardeningknowhow.com/ornamental/bulbs/crocosmia/crocosmia-bulb-care.htm