Below is a screenshot with the numerical steps on how to find Salesforce Custom Contact Fields. It is suggest to use Salesforce Classic to best access the below information.
As a Salesforce Administrator, the steps are as follows:
1. Select setup / gear icon in the top right.
2. Under the Build section on the side panel, select Customize.
3. Select Contacts.
4. Select Fields.
Inside Fields, at the bottom you will see "Contact Custom Fields & Relationships". Selecting New will allow you to add your desired custom Fields.
The link below will direct you to the Salesforce Help Article with the steps on adding / creating a custom Field.
Salesforce: Create A Custom Field.