Go to google maps, navigate to the general area you want to map out. Click the "My Places" button, then click the red "CREATE MAP" button. Give your map a title and description, and make sure it is listed as public. Add pins, shapes etc to your map.
Step 2: Link you map.
Make sure the map is centered and zoomed the way you want it, and click the link buton (looks kind of like a chain, in the top of the left column). DO NOT click the Short URL checkbox. Copy the link into a post. (NOT the embed code, just the normal link in the top text box)
Step 3: Set up the tag.
The only part of the link you actually want is the part that comes after the question mark. Remove the rest of the link, and enclose the part that comes after the '?' (not including the question mark itself) in gmap tags as below. You may have to go back into google maps and adjust the zoom level. All embedded maps will be set to satellite mode by default.
This will result in: