1. Go to Admin and click Field Management.
If you want the fields to be kept in sync with your CRM, create them in the CRM and they will automatically be created in Marketo.
2. Click New Custom Field.
3. Choose the field Type. This will change how it is rendered in smart lists and forms in Marketo.
Check out the Custom Field Types Glossary.
4. Enter the Name as you want it to appear in Marketo.
Pick the Name and API Name carefully, as renaming fields can be difficult and in some situations not possible.
The automatically generated API name is used by the SOAP API and other backend processes.