You write an invoice when you want to receive money for a fulfilled work. This happens mostly on the basis of a service contract. In example, you sign a service contract with an organization for performing a project management training as a facilitator. After the training you send an invoice. When accepting the invoice the organization will pay out the agreed salary or honorary.
 Fiscal documentation
For your personal financial documentation it is very important that you archive both, your contract and your invoice. Your fiscal authority will possibly ask you for those proofs.
 Professional competence
How you write invoices is as well a statement regarding your professionality. Some people use simple templates that some organizations show them. Others develope personal templates in their corporate design. Especially professional freelancers know, that good invoices support the impression of their good work. Therefore invoicing is a professional competence. Keep in mind: Your successful performance is possibly forgotten after two or three years - your invoices are mostly archived for a longer time (often five years and more).
 Structure of an invoice
How to write an invoice? Take this proposal as a basic frame. It fits especially for invoices, addressed to orgganizations inside the European Union.
[Personal Data] Petra Exampelova 10 Example Bvd 99999 Exampelov Exampelia Email: firstname.lastname@example.org Phone: +44 1234 56789000 Tax number: BCD-123456789 (when you are inhabitant of the European Union) Exampelsk, 2013/02/30] [Address of the institution which receives the invoice] Example-Organization Example Street 11 11111 Exampelsk Examplistan [Subject] Invoice for: Texts for Example Toolbox Dear Ms/Mr Example, I hereby invoice the provision of the following service "Texts for the Example Toolbox" [description of your work], provided during the period 2010, June - August [period of service], which amounts to [Amount and calculatory basis] XX.00 EUR (XX day rates of xx EUR x 8 days).[Please here to include on what calculatory base you come to this sum] I wish for the amount to be transferred to the following account [Account Data] Account holder: Petra Exampelova Name of bank: Cash Cow Ltd, Exampelsk BIC: 1582824 (or usual national account information) Yours sincerely [Original signature] Petra Exampelova