Step #1: Copy/paste
As always make sure to Download a copy of your template and save it to your computer. Or if you don't feel comfortable editing your template create a test blog and try it there and then do it on your blog and you're sure you won't damage anything. If you look at the top of this blog you shouldn't be able to see it because I've inserted two lines of code into my template that hides the navigation bar until someone hovers their mouse over the area. This is the code you can copy/paste into your template:
You have to place it between the <head> </head> tags in your template. A good place to put it that is easy to find is right after this line in your template:
/* Use this with templates/template-twocol.html */
Scroll down a little bit from the top of your template to find the line of code. Here are two scrceenshots to help you with the placing of the code.
Step #2: Preview & Save
Once you've pasted the code into your template click "Preview" and if you did it right you should get a successful preview of your blog without the Blogger navigation bar displaying. After that you can click save and continue with your blog.