Bookmarking involves going to the main map and clicking on places you would like to have saved. To bookmark, you could click on the area and go to the left hand corner of the pop-up. There, you should see a little icon that looks like a bookmark. When you press that, you can choose what the bookmark is for, those options being: "favorite," "friend," or "enemy." You can also edit the title of the bookmark to make more sense for you.
After you bookmark something, those coordinates are saved and you can access them at any time by going to the same icon at the top of the screen on the main map and pressing it to bring up all of the places you bookmarked. Initially, there are only ten spots to bookmark, and when you run out of room, you have to pay gold to get more spots.You can bookmark other cities, resource tiles, monsters, or just random places on the map.