Step 1: Prerequisite:

While creating a new FBA shipment SellerChamp manifest, within the manifest settings, under the FBA Shipments Workflow option, choose No, I will create FBA shipments after I've scanned all my item.

Step 2: For the items that will be case packed, once the item has been added to the manifest, click the edit item button
Under the Selling Details tab, for the Case Packed option, select Yes.

You'll notice the fields changing / populating, asking you to enter the:

Once the information is entered, click on Save Item and you're done.