Why you might want to install code:
There are plenty of websites suggesting to tweak your blog. A few these tell you how to use standard Blogger tools, but many have written code that does something useful that Blogger doesn't currently provide.
Also, there are lots of tutorials about how to do things with HTML, CSS or Javascript - no matter what "development environment" (Blogger, Dreamweaver, etc) you're working in. For example, I recently wanted to put tables with scroll-bars into the posts of one site, and, after a bit of googling found a method that works across all the common browsers. (Well it worked when I tested it first - sadly it wasn't working in Chrome when I finally finished loading the content, but that's another story!).
I'm generally wary about installing things from 3rd parties. But there are times when it's a good thing to do because the benefits for your blog outweigh the risks - for example, I'm happy to install code from AdSense, Amazon Associates, Chitika, PayPal and various other well known brands.
How to install code:
Two previous articles describe how to install 3rd party HTML into your blog, and how to add CSS to your blog.
The only other type of code that you can add is Javascript. To add this:
1 Edit your template, in the usual way.
2 Find the </head> statement
3 Put the code immediately before </head>, using this format:
<script type='text/javascript'>//<![CDATA[THE CODE GOES HERE
//]]></script>
4 Click the Preview button, to check that the blog loads and looks ok.
5 If it's not ok,choose Clear Edits and try again: check you are putting the code in the right place.
6 Once you're happy, Save the changes.
Related Articles:
Adding HTML into your blog,
Installing extra CSS rules into your blog.
Planning changes to your blog - in private
Showing Presentations in your blog
Is code from a 3rd party safe to install on my blog? (coming soon)