How do I customize permissions for a specific user in a specific SharePoint subsite?

1. Go to the Site Settings page.
2. Select 'User Permissions' from the 'Users and Permissions' category.
3. Select 'Grant Permissions' within the ribbon.
4. Choose the user to whom you want to assign the permission.
5. Select the permission type in the permission dialog box.
6. Click OK to confirm and save changes.