In order to add a purchase order to a supplier, find them through the suppliers tab either as a 'Pinned supplier' or using the 'View all suppliers' function.
If the supplier is pinned, you can simply click 'New PO' from their profile:
Otherwise, you can find them in your list of suppliers and open their profile by clicking 'View'.
From their profile, you will be given a list of all their existing purchase orders as well as the option to add a new one:
From the following page, you will be able to use the tables provided to fill in the details of the purchase order itself. You can also add any notes you wish to include by expanding the 'Advanced options' tab at the bottom of the page:
If you need to make amendments, once completed, click 'View' to open the purchase order itself. Click 'Edit' in the top right corner. You will now see a page with the fields you can edit. Change this information as required and click 'Save'.