The new interface has all the same elements as the standard Opportunity Contact Role.

To add additional contact roles, select Edit Contact Roles.

Click Add another soft credit link to add more contacts. You may only add contacts, not accounts.

Use the lookup window or type in the contact name.

Select the soft credit role. These options are selected in the standard Opportunity Contact Role editor in the setup interface.

1. Select whether this contact gets full or partial soft credit
2. If partial credit, enter the amount or the percentage
3. Select whether the value is the amount or percentage. Click Total must reconcile to opportunity if you want to receive an error message if all soft credits don't add up to the total of the opportunity. This makes it easier to enter a large number of partial credits on a single gift without worrying that you might have a math error.

When done, click Save.

You can only edit soft credits in the contact role editor. If you want to change the primary donor, click Set Primary Role

This will take you to a standard contact role editor where you can select a different contact as the primary donor role.

When you click Save, you will see this information updated in the custom section on the opportunity.