I need some detailed how to instructions.
What size should the header be?
How do I upload it?
Where do I upload it?
How do I use filezilla to up load it?
I just need some detailed advice... Thanks
Just do a search for "logo", "replace logo", "my logo", etc. doing so has been discussed so often here that the answer to your question has probably already been answered many times over.
It wasn't easy for me either to get an answer to the question you have asked.
#1 Know your FTP account and connect to it using Filezilla or similar.
#2 Know your site's file & folder structure. In other words learn were & how your web site content is stored on your site's server.
Now, there is no answer for the size of a logo/banner. It is what ever you make it and how it actually uprears to others depends on their browser and screen settings. There is NOT a defined size for any header banner!! If it looks good to you, on your screen, go with it until you get site visitors rampantly complaining about it.
The trick is to get your MyBB installation to 'find' your custom Logo/Banner and to show it consistently on your forum's pages.
I was given that answer as to how to do that a couple of years ago but --- after much searching here via a separate tab --- I was unable to find the answer given to me.
Sorry, but I do recall that it had something to do with the "Theme", "CSS" that had to 'refer', rather specifically, to a file URL location.
Hmmm...
Good luck!! ---- And welcome to the 'headaches' of amateur web-mastering.