Contact Custom Fields are extremely important and can supplement the amount of information you store within your contact records.
But how do you navigate to add them on?
A: Simply go to Contacts and you will see the Custom fields tab at the top of your screen.
Now click on Add new Contact custom field and you can add them on there.
Hint: if you are about to import contacts in bulk make sure that you have added all your required contact custom fields to match your spreadsheet. When you then match your fields across you will see your contact custom fields beneath the default fields.