number for invoices and forms automatically increasing by 1 each time I open the template

[Fedjwick]

How can I establish a number for forms or invoices that AUTOMATICALLY will increase by one (1) each time I open the template to save a new document? As is I have to manually change the order number or invoice number and sometimes I forget which leaves me with more than one order or invoice with the same number. Seems like an easy enough task but I can't find a way to do it on my own. Need specific step by step instructions.