Login to your 1Shoppingcart account and click on "Contacts" then "Custom Fields"
On this screen you'll see a list of your current custom fields. To create a new custom field, click on "Create New"
Now, give the field you're creating a name for you (label) and a name for the computer (short name).
You can probably disregard the "Display on Checkout" and the " Display on Mobile Checkout" options. Hit save when you're ready.
By default, it will be set as a text field but you can change that to one of the other field types. Learn more about the types of fields you can create here: https://amazingsystem.zendesk.com/hc/en-us/articles/203342697-What-type-of-custom-fields-can-I-create-
Now the next time you create a form, you'll see the new custom field you created on the "fields" tab ready for you to use.
The field type can be changed after the fact. So for example, if you realize that a text field isn't enough space, you can change it to a "text area". You may have to regenerate your form for the field to change on your website.