Install like any other D7 module.

After installing the module you'll want to set permissions on who can
purchase via PO by using Drupal's permissions system.  Search for 
"purchase order" and decide which role(s) should be able to use purchase
orders.

The PO number will show up when viewing and editing orders in the admin
interface automatically, but to make the PO number show up in the 
invoice(s) shown to the customer, you'll need to edit the template(s) 
that the uc_order module uses.

These are usually located in sites/all/modules/ubercart/uc_order/templates

To add the PO number to a template, you can make a call to the function
uc_po_invoice_po_number($order_payment_method, $order_link)

Note that the function returns the html for the PO number, but doesn't 
display it (in case you wanted to modify it first or something).

To display it just echo it;  something like this in the template should
do the trick:

<?php echo uc_po_invoice_po_number($order_payment_method, $order_link); ?>

For an example, see templates/uc_order/uc-order--customer-with-po.tpl.php.
