Banner makers needed <3

So I don't really know how banners for stories work or how you make them
or even how you put them on so if someone could help me that
would be much appreciated!!!
Even if you don't want to make one for me...can someone just tell me how?
So I can experiment and make one for myself :)
I really really wanna make one for my story but I don't even know how
to put on my page!

anyways,
Thank you so so so much.
It would mean a lot if someone told me how!

Check out the story I'm making it for:
Just Once