The portal users can ‘edit’ any existing records if the editing rights are given. 
Click on the Submit button once the records are edited.
The portal users can add new records if the adding rights are given. As per configured form the Page Builder, the fields will appear to insert the details. 
You can select “Account” or “Contact” as per the Lookup records.
The portal users can add the product if the product selection rights are given for the ‘Order’ object. 
The portal users can select the Price list from the Price list look up records. 
After inserting all the details, the portal users need to Submit the records to save the details.