Begonia corns are beststart them in January February in a dry soil based John Inness compost lightly watering them every week on a cool window sill in a cool room maybe a hall way etc. Then In April I would move them into a glasshouse with frost protection using horticultural fleece and a small heater if it turns cold over night.
Also as the first shoots come when they get to 4 to 6 true leaves you could try pinching to tops out to create a few softwood stem cuttings and this will also help the plants to bush out and become more compact