literally just did it last week. Preheat oven to 180 - 200 degrees CONVECTION. Pop them in there until there puffy and soft. 5-10Min
Ideally you should have toe caps on, but you can always make youre own out of old socks (cut the toes off of them).
I know theres more specifics that people will banter about, but my liners arent exactly new so i dont mind