Click on the world map, choose the location you want to save, click on the star sign, and choose the category.

To find your saved locations while you are on the world map, tap the file icon at the bottom of the screen while you are on the world map.

also, from where you can delete a saved location, tap on it, swipe from right to left, then click on the delete icon

If you delete the game, these saved coordinates will be deleted, so please save or take a screenshot for them before doing that step...