Field marker is a great function in the game and is used at the map page.
It makes it easy to save a location you want to remember but keep in mind that you can't make them visible to players in your kingdom if you are a govorner or duke. How ever if you are a king you will get the option to do so.
Just press on a village/an empty spot you want to mark and take the option at 6 o'clock called Mark field.
When you are in the mark field page you have two options, either choise the "Edit marker" or "All markers"
In the All markers you can choise to delete your markers.
If you want to add a new marker easy just press on the Edit market tab and pick an option that fit the best.
Mark field, Mark player or Mark alliance.