From the menu on the left of MiniMRP program window select the option New Customer Order.
Then select a Customer from the drop-down list - arrowed in the picture above
The adrop-down list also contains the option to Create a New Customer - ie if the Customer you want to use doesn't exist yet
Once you've selected (or created) the customer the new order form appears - see below
Enter the customer's purchase order number - if there is one and then click the button in the bottom right corner of the window to begin adding items to the order. - see next page