Make sure you have backed up your template before installing a new one.
(You can also install a XML template).
You have to choose a Html Template
Copy the code (Ctrl A: Select All, Ctrl C: Copy)
Go to your Blogger Control Panel - Edit HTML
Revert to Classic Template (You can return whenever you want).
Paste the code into the Template area (Ctrl A: Select All, Ctrl V: Paste)