First of all you would need to create a new position in the template by editing the template code so that the position is in the right place on the template itself
You would then need to create a new custom module or use the existing banner module in joomla, add the banner details in there and then select the new position you’ve just adding in the template.
If the position isn’t displayed in the position selection box – don’t worry, just type in what you’ve called the new position and the banner should show up don’t forget to enable it).
Hope this helps you to get started. I guess the next question is – what part of the template do you need to edit???
😉