First you need to make a banner. You can do this however you want. A good way would be by making an artbook and then prt sc and cropping this artbook. (Just like any other image from MSP).
Now that you have made a banner, you need to put it as your banner on your blog.
1. Go to the blogger dashboard.
2. Click the drop down arrow for more options.
3. Go to template.
4. Now go to layout.
5. Click the small edit button in the edge of your blogs name.
6. A window should have opened.
7. Click the add image.
8. Find your image and add it.
9. Now it is your banner. All you have to do is click apply to blog (this is at the top of the page).
I will add some prt sc to help you with the steps soon.