To begin with, you need to create (or update) the Custom Field you'd like to use, by clicking on the Profile Icon in the top right corner, then on Settings, then click on the Custom Fields link in the left menu bar.

If the custom field already exists, make sure it's enabled for Locations, by clicking on Edit, switching the Enable toggle on, and clicking Save.

If it's a new custom field, then click on the Add button to create a new one.

Start by giving the custom field a name and selecting what type of field it should be.

These are the types of custom fields you can create:

To add options to a Dropdown List custom field, you just need to fill out the Dropdown List Content field with the options separated by a comma, like: Green, Black, White, Red

You can also choose if it should be a required field or not.

Then, scroll down on the same page and select which data types you'd like the field to be enabled for.

It's enabled for all data types by default, but you can disable it for all the data types that you won't be using the custom field for. At a minimum, you need to leave it enabled for Locations.

Once you save the custom field, you can see that it immediately shows up in all your locations.

In order to remove a custom field from a location, you just need to disable the custom field for Locations in the Custom Field section in the Settings.