You can start customizing a form by opening it in design mode process in 3 ways:
1. From the Main App Page
Specify a list and select the form you want to customize. For more information, see Main App Page.
2. From the List View Page
Select the Customize Rich Forms option on the List View ribbon.
3. From the Form Page
Click Edit Page action in the Settings menu of the form page.
After you perform one of the above actions, the form is opened in design mode and you can start customization using the Lightning Forms Editor features:
Field
Group
Tabs
Toolbar
Rich Text Box
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