Go to the main section Orders
(or Documents - Purchase orders on Website)

Click on the + icon in the upper right corner

Add a client. Saved clients will appear in your list. Click on + to create a new client. You can also "Choose from contacts" to create a client from phone's contacts

Add a product - choose from the list of your saved products or create a new product

Edit or add Details (Date, Discount, Shipping, Currency, Language)

Save