Hi
What aspect of invoices do you not follow? The business angle or the access angle?
From business PoV there are certain must haves - such as company name and address, name and address of recipient, VAT number if applicable, invoice date. There is almost always an invoice number.
The invoice itself then needs to show details of what the invoice is for, the amount owing, and VAT due and the rate of VAT applied (which needs to be shown separately), any discounts applied and then a grand total of the amount due.
Sometimes then also have a payment due by date. May also include terms and conditions.
Look at any bill you have received and you will see the sort of thing.
The access angle involves drawing together the items for invoicing, making the necessary sums, allocating customer name, allocating an invoice number and printing off the report.
You would also tend to have a check field on items to show that they have been invoiced or perhaps a field to hold the invoice number for cross referencing.
Does this help?
Am quite happy to explain more if you want.
Good luck
Malcy