Select Roll-Up Summary and click Next. Enter the details accordingly.
Choose Opportunities for Object to summarize. Select the sum of Roll up type, in which the Field to aggregate is Quantity.
Click Save. Your roll-up summary field is created.
The PDF shared above will help you with a step-by-step guide to creating roll-up summary fields. And if you need any help, our certified Salesforce experts will assist you in the best way possible.