First of all you need to create the map...
Sign into your Google account, and head to Google Maps.
Click on the "My Places" option at the left of your screen, under the Google logo. Then hit the red "Create Map" button.
Give your map a title, and if appropriate, you can add a description. At this stage you can choose whether you want your map to be visible to the public, or private for just you and who you share it with.
When your map has been created, add your first location. Select "Save to map". If you'd like to add multiple locations, search for another location, then click "Save to map" and choose your map from the drop-down menu.
Click the link icon at the top left of your screen. Grab the iframe embed code and paste it into WordPress using the Text tab on your page or post.