Opportunity custom categories and fields are created via the Control Panel settings (permissions required to access the control panel).
Go to Control Panel > Opportunities > select Edit Custom Fields on the right of the screen.
Similar to Contact custom fields, Opportunity custom fields are recorded underneath categories. The Uncategorised category is a system default category available to use if needed. Categories display as tab headers in the Add Opportunity window modal.
Click the Add button in the top right corner of the screen to add a category. Define the category name according to the business requirement.
Use the same Add button to select a custom field type to add within a category. A combination of similar or different custom field types can be used within each category type. Choose one of the Text, Number, Select, Date and/or Time, Lookup or Multi-Select field options from the Add drop-down. Then use the category drop-down to assign that custom field to its respective category. By default, all custom fields are assigned to Uncategorised if one is not assigned.
Use the option to drag and drop custom categories and fields to arrange their placement. The Add Opportunity window will display categories and fields in the same order as laid out in the Control Panel settings.