Issuing an invoice is one of the most common tasks if you are self-employed or have a small business.  

If you’re just starting out or want to make sure you’re doing it right, here’s a handy guide to the main things you need to know about how to make an invoice and which details you shouldn’t overlook.

What should an invoice include?

An invoice is a legal document that justifies a commercial transaction. It must contain a number of minimum criteria in order to be valid:

• Invoice and serial number: Each invoice must have a unique, sequential and serial number (e.g. 2025-01). If you issue different types of invoices, you can use different series to organise them better.
• Date of issue: The day on which the invoice was created. 
• Sender and receiver data: Name or company name, VAT number, tax address and any other relevant information. 
• Description of the products or services provided: Specify clearly what you have sold or what service you have performed. Add units, unit price and subtotal.
• Taxable base and tax rate: It indicates the amount before tax, the VAT rate applied and the personal income tax withholding if applicable (common for some self-employed).
• Total amount: The total amount payable, including taxes. 
• Form and terms of payment: You are advised to specify how you expect payment to be made (bank transfer, direct debit, etc.) and the deadline.

Recommendations for good invoicing

In addition to complying with legal requirements, there are a number of good practices that can help you keep your invoices in order and avoid problems:

Correct number sequencing

You must use sequential numbering and always in ascendingorder. If you use letters, they must be in capitals (e.g. A2025-01). You are also advised to use a hyphen or other special character instead of blanks. You should also take care not to skip numbers or duplicate invoices.

Take care of the presentation

Although it is not mandatory to use an elaborate design, the invoice should be clear and easy to read. You can use professional templates or invoicing software to streamline the process and maintain a consistent and professional image.

Archive and keep copies

Keep all invoices issued and received for at least four years. Ideally, they should be organised digitally, by date or client, so that you can access them quickly when you need them.

Always issue an invoice, even if the customer does not ask for it

In many businesses, it is mandatory to issue invoices. Even if you work with private individuals and they do not ask for it, you are advised to do so in order to record your income and fulfil your tax obligations.

Invoice on time

Issuing invoices late can give an unprofessional image and also affect collection. It is best to do this as soon as you finish the job or deliver the product.

Specify the taxes clearly

Don’t forget to correctly apply VAT and, if you are obliged to do so, personal income tax withholding. Many self-employed people are subject to both. Make sure you use the current rates and break them down correctly.

Add clear payment terms

Indicate the account number, the method of payment and the deadline. If you usually work with deferred payments. 

Common mistakes to avoid

Learning how to invoice correctly also means knowing the most common mistakes:

• Leaving out some mandatory information (such as the customer’s VAT number). Always check that you have filled in all the fields. 
• Failure to apply the correct taxes. It is essential to be up to date with the various taxes.
• Repeating invoice numbers. You should never use the same number twice or skip a number. 
• Not indicating the method or term of payment.
• Forgetting to send it to the client or sending it late. You are advised to always send the invoice as soon as the work is completed. 

Tools that can help you

You don’t need to do it all by hand. There are many tools that make it easier for you to manage your invoices: 

• Online invoicing programmes: Many allow you to create invoices in just a few steps, keep track of collections and generate reports. Some have free versions that can help you choose the best option for your needs. 
• Spreadsheets: If you prefer something simpler, you can use Excel or Google Sheets with ready-made templates for invoicing. 
• Mobile applications: Ideal if you work from different locations and need to issue invoices from your mobile or tablet. 

Knowing how to make an invoice is not only a technical issue, it is a fundamental part of the daily management of your activity as a freelancer or entrepreneur. Getting it right from the start will save you time, avoid mistakes and reinforce your professionalism in the eyes of your clients. 

With good organisation, the right tools and attention to detail, invoicing will be an easier and more efficient task.