To create new custom field, please follow the easy steps below:
Click on "Lists"
Click on list name of your choice
Click on "Custom fields"
Click on the button "New custom field" at the top right
In the popup you have some "mandatory" fields
let's say you want to add the custom field "city"
Label is mnadatory, you add "City"
Slug: in this field you need to enter "one word" no space allowed, you can add for example: "CITY" or anything you want
Choose wether it's required or not
In the "Type" you can choose the type of the field. In general we choose "text" by default
Once the 4 steps below are done, click on the button "Add"
You can see the custom field you just created in the list of custom fields. You can also remove any custom field you want.