To create a map, you need to first register to the website by going on the registration page.
Once this is done, click on the map button in the menu and you'll see a link in the submenu named Create my map.... Click on it.
There, you'll be able to choose the game for which you would like to create a map. Fill out the name, choose the layer and position the map as you wish to show it. You can for example zoom on Los Santos if you wish to show this part instead of the whole map (the map will still be movable). You can also decide to check the Public map button or not. This will put your map in the user map list which can be seen by everyone.
You can put a marker on every map you want but you can only save them on your own maps.
To enable this functionality, you will need to click on the marker button which is at the left of the map. Once it is enabled, simply click wherever you want on the map to place the marker. You can adjust the position by drag'n'dropping it.
You can remove a marker you just placed by clicking on the Remove button or by disabling the customer marker functionality.
If you wish to edit or delete a map you made, you can to this by going on your maps. To access to this part, you need to click on your username on the right of the menu and select My maps. You'll be able to do this from this page.
Or you can also directly edit or delete a map from the map itself. On the side bar, just below the map name, you can see two icons for edition and deletion.
If you wish to edit (change name, category or position) or delete a marker you placed. You need to be on your map and click on the Items button in the menu. Find you marker in the list and edit/delete it.
Or you can also edit or delete a marker by clicking on it on the map. You will see two buttons on the side bar, one for editing the marker, and the other one for deleting it.