I would not recommend that you use a self designed XML template until you are familiar with blogger as you can end up with many errors.
However if you want to proceed what you need is to
1. Go to your blogger dashboard
2. Find the button that says 'View Blog' BUT DO NOT CLICK IT
3. To the immediate left of that button is a pointing down arrow - click on that
4. Select 'Template'
5. A screen will appear with two options 'customise' and 'edit html'
6. Click on edit html
7. A warning will come up
'Directly editing HTML may affect the way some features work and is only recommended for advanced users. Editing template HTML may unpredictably affect other Blogger features. We recommend using the Template Designer first, where you can Add CSS under the Advanced section. '
8. You will be presented with three options 'Proceed', 'Open Template Designer' and 'Cancel
9. Select 'Proceed'
10. Your current code will appear and I suggest that you copy and paste that template as a back up first.
11. You can now delete your current code and copy and paste your own xml template into the page and preview it before you save it
There is no way to upload a file (at least not a straight forward way that I know of)
I have modified my template using this method and the back up is useful if you mess up (which does happen).
Again, I can only stress to you that it is probably best to work with blogger templates rather than a self designed or downloaded one until you get used to it.
Good Luck!
