I gotta be honest...when I first got started, I was very leery of playing around with the HTML of my blog. I was so intimidated. I was worried that I was going to mess something up and lose my current template. So, what I did was create a separate blog whose sole purpose is to try new things out on. I now have a safe place to play!
Here's how to create a practice (test) blog:
Go to your dashboard. Click on 'create a blog'. Personalize it to your liking. I would recommend selecting 'minima' as your template. Most of my blog tutorials are based on my experience working with a blank minima template. You'll need to select another template to begin with, but will be able to change to minima easily once the blog is created.
Wanna hide your test blog? Simply go to your dashboard, edit profile, click on choose blogs to display and uncheck the box next to your practice blog. If you really want to keep people from eyeing your practice sessions, go to settings-->permissions and change your 'blog reader' setting to "only blog authors ". You could also change your basic settings for Blogger not to list your practice blog on search engines or add it to blogger's listings. Within minutes, you will have a secret place to practice your new skills away from the eyes of your readers.
You can experiment with fonts and colors, play around with your margins and save to your heart's content, worry free! When you get everything looking the way you want it, open your real blog and SAVE A COPY OF YOUR CURRENT TEMPLATE. Replace the portion of the code you want changed with the code that you perfected in your practice blog. Then preview (this button is located at the bottom of the 'edit HTML' page). If something doesn't look right, 'clear edits' and go back to your practice blog to play some more, or look carefully at the top and bottom of the code that you copied and pasted and make sure you didn't leave anything out- like an } or something. BE CAREFUL. Do not hit "SAVE" on your real blog until you are certain that you like what you see in your preview.