To create a purchase order, the first thing you have to do is to assign the job in question to a supplier. For that, you have to click in the job on the Outsourcing tab. To know how to outsource a job to a supplier, see this article: https://help.lsp.expert/en/articles/1168599-how-can-i-outsource-a-job-to-a-supplier.
To create a purchase order for the supplier, click on 'Create a purchase order' in the details of the outsourcing. (To access the details of the outsourcing, click on on the Outsourcing tab of the job.)
The PO has now been created and has been added to your list of POs. The Actions button allows you to generate a PDF document and to send the PO per email, among other options. To edit the details of the PO, click on Edit in the Overview panel.
You can decide on the format of the purchase order number. The system will try to increment it for your next purchase order.
To change the status of the PO, simply click on it (in the details of the PO or simply in your list of POs).
If the PO is defined as Sent in the client's system, the supplier can also see and download the PO from the supplier portal. To know how to use the LSP.expert supplier portal, see this article: http://help.lsp.expert/en/articles/3340222-getting-started-with-the-supplier-portal