Image Hosting for Your Background
Once you've downloaded your Blogger Blog Background, you need to put it somewhere where your blog can access it. You need image hosting. I chose Photobucket. It's free. You need to register, and upload the image there. Once you've done that, choose 'Direct Link'. Clicking it will copy the link. You'll need it later. Leave it open in its own window so you can come back to it later.
Create Your Blogger Blog
Now you need to create your Blogger Blog.
Choose a starter template, don't do anything here, just press the orange 'Continue' button
Your blog has been created. Choose customize how your blog looks.
You end up at the Blogger Template Designer.
Choose 'Back to Blogger' in the top right corner.
Choose Edit HTML
Scroll to the bottom and choose 'Revert to Classic Template'
Press 'OK' to revert
Choose 'Pick Classic Template'
Choose the 'Minima' template
Choose 'Edit HTML'
Find this piece of code:
It's located near the top of the template, so won't be too hard to find. Click the pic if you need to see it better.
Replace with this code:
background: url(YOUR PHOTOBUCKET DIRECT LINK) no-repeat center fixed #FFFFFF;
Go grab (copy) your 'Direct Link' piece of code from your open 'Photobucket' window or tab.
Replace YOUR PHOTOBUCKET DIRECT LINK by pasting in the link you just copied. Those brackets need to stay in the code.
Scroll down and 'SAVE TEMPLATE CHANGES'
Now you can view your blog! (Click pic for a bigger view).
Of course, you can play around with the HTML to change font colours and sizes etc.
If you use these backgrounds I would love to see your blog!