You now create custom fields for leads and deals centrally in Setup under Layout ↗. There you drag a field type from the palette to the spot you want, group fields into sections and hide fields without losing any data.
The full guide is in the article Customizing the layout: sections and custom fields.
In short
- There are nine field types: text, number, currency, checkbox, dropdown, multi-select, date, date and time, and link.
- Every field has an API name. It is generated automatically from the name and never changes, even if you rename the field.
- In the lead detail view your custom fields appear in your sections and under Additional data.
- During the import you can create a CSV column as a new field or map it to an existing field, see Creating, importing and enriching leads.