How to care for a severely sprained knee? Is a cast necessary

I twisted my knee on a ski trip.theres lots of swelling.i can’t put pressure on that leg and I can barely walk.most of the pain is under the knee on the outside of the joint area.can I treat this at home or do I need to see a doctor

Apply ice wrapped in a towel or cloth to the injured area for 15-20 min. Icing is most effective in the immediate period following an injury. The effect of icing diminishes significantly after about 48 hours.
Keep the injured body part elevated above the heart while icing – this will further help reduce swelling.
Consult doctor asap