To create a receipt for items associated with a PO, you'll want to navigate to the "Receive Items" section of the mobile app. This will bring you directly to the receiving form.
You'll want to first select the receipt type: item or expense (services). Selecting one or the other will produce a receipt specific form, so it's key to select the correct receipt type.
You'll then want to select the purchase order number found in the packing slip or associated to the service. This will then populate all the receivable items or services from the PO. You can adjust quantities as is necessary, as well as include an image of the packing slip if you are receiving items.
When you are done filling out the receipt form, just click on the "log receipt" button at the bottom of the form screen.