Meris...You can buy window sealing fits from hardware stores. Simple to put on the window and definitely will work!
Make sure there is enough spray foam around the windows, and you can also caulk all the joints. It depends how old your windows are, but you could also use a product like this to keep out the drafts: https://amzn.to/2ZeuGQG
Hi Merlis. One of the best ways is to caulk around the windows. Use window caulk only. Next, use heavy cloth drapes or canvas as drapes on the windows to seal out the cold. On sunny days you want draw the drapes/canvas back to let the sun warm the room.