Create a UserControl as described in the hello world article and then use the <portal:PlusOneButton inside your UserControl and add the UserControl to any content region on the page just like any other feature.
Or you could just add script and markup manually in the html view of the editor in an html content instance.
The editor probably won't let you use the <g:PlusOne element because it is not valid html, but you could use the html 5 syntax.
<div class="g-plusone" data-size="standard" data-count="true"></div>
Hope that helps,