Make sure your invoice number or reference is always unique, on that way when the tax man or the client want to query an invoice you can quickly locate it using it's unique reference number.
Pay extra care to this one as it's a legal requirement in most countries.
You should make sure you are displaying your business name, address, and any other information that will make it easy for your customer to contact you, and have a good idea of who is your business.
The minimum information should include your business name (or your name if you're a sole trader) and address.
if you are a limited company, make sure to include your company registration number, and any other tax reference numbers such as VAT (Value added tax).
Add Your Logo
Although this is not strictly necessary, you should always carry your business identity across all your communication.
That will give your client confidence, and show that you are a professional who cares about your image.
Add your invoice items
A typical invoice will have one or more line items.
Make each line item description coherent, and easy to understand, on that way avoiding unnecessary queries and delays to your payment.
In the UK you must only charge VAT if your business is registered to do so, and that is probably similar in your country. Feel free to to skip this section if you're not intending on charging VAT.
Charging VAT will include a few levels of complexity, to which you must adhere to. For instance, not all products and services are allowed to charge VAT, make sure yo check this information before issuing a VAT invoice.
Additionally you will need to produce reports and pay your government tax institution the VAT collected from your customers.
Different rules applies for certain types VAT schemes. Make sure to get all information necessary to ensure your invoice is compliant with your local legislation. This information is best obtained from an accountant, or your tax office website.
Display additional information
More often than not, you may need to display additional information such as payment terms, and bank details.
You should have a notes section within your invoice, which you replicate across all your clients.
This would be a little snippet of helpful information, again giving your client details before they even asked you, thus saving precious time.
Keep an archive
Last but not least, it's normally a legal requirement to keep good records of every single invoice you generate.
That can be done in the form files stored in your local computer, cloud drives such as google cloud, or preferably you could use an online software such as Fast Invoice to take care of this aspect, and all others covered here.