The WPGeo plugin must be activated for your site by an administrator, and needs a Google Maps API key. You can tell if it is active by looking at the Plugins page on your site’s Dashboard, or if this symbol appears in the visual editor:

WPGeo Control Panel

Using this control panel, you can find the general location of the map you want to insert, and then zoom in to set the precise location of the marker. If you use the other controls,  you can set the title of the pin (default is the title of page or post) and customize the zoom and appearance of your map. Your readers will see the map you created in your page or post, and have the option to open it in Google Maps for further exploration.

A blog or a website can have one page that combines all of the GeoMaps used on the site pages and posts into one map that summarizes all locations on the site. To insert such a map, you need the shortcode:

Using the mashup map, a reader can select a marker on the combined map to be taken to the page or post that discusses that marker.

More short codes and instructions can be found on the plugin page for WPGeo.