How do i do that?
First get rid of the rust with steel wool and dry well
Take shortening or oil with a high smoke point on a paper towel and rub well all over the pan, inside and out including handle. It shouldn't look wet when done, should be a very thin layer
Put upside down in oven on highest temp for half an hour. Put a pan underneath to catch any drips.
Let it cool enough to handle, then repeat. Do this a few times to build up seasoning before using.
When washing, make sure to dry it fully and immediately so it doesn't rust again.