Firstly check that your customer has a valid VAT number and add that to the customer profile.
Both the buyer's and the seller's VAT number needs to be on the invoice. You can easily check the buyer's VAT number in the service provided by the EU (VIES VAT number validation).
Your add your own VAT number under My account -> Company information.
1. Go the Invoicing tab, and choose Create Invoice.
2. Choose the Customer
3. Choose the Invoice type: Sales of services to another EU-country from the invoice editor to the right
If needed, you can choose another language or currency for the invoice
When you choose another currency a new field will appear showing the conversion rate.
The buyers VAT number is added to the Invoice footnote after the legally required text "VAT 0% Intra-Community supply of services".