You have to follow 3 easy steps:
- Create the widget
- Add it to your Blog
- Add an Install-button to your Blog
Go to WeatherReports.com, scroll down and click Add WeatherReports To Your Site.
In the next screen, enter your city, select the colors and style of the weatherbox. After entering all your selections, you will go to the next screen automatically. Take a look at the preview, and select and copy the HTML-code.
Step 2: Add the Widget to your Blog
Add an new HTML page element to the sidebar of your Blog. Paste the code that you copied into the page element, and set its title to "Weather Report".
Don't save it yet, but go to step 3.
Step 3: Add an Install-button
Now we are going to add a button, that installs the widget on your visitors blog when it is clicked. In fact, we'll add a form with several input fields, but only the button will be visible. Here we go.
<form action="http://beta.blogger.com/add-widget" method="post">
<input value="Weather Report" name="widget.title" type="hidden"/>
<textarea style="display:none;" name="widget.content">
<input value="Install to Blog" name="go" type="submit"/>
The text between the <textarea>-tags is the html of your widget. Please notice that each < is replaced with <, and that each > is replaced with >.
Now save the page element and view your Blog.
For an example look here.
If you want more information on how to add third-party widgets to your blog, take a look at these help pages.