You have several options. Myguichet : You can file it on myguichet. To do this, log in to your myguichet account and submit your Form 100 and the attachments. You can then sign directly on myguichet. By post : Check your tax return, print and sign it. You can then send the form and your attachments by post to the relevant tax office. The address of the relevant tax office is shown on the form (first page top left) and in the "Result" form on taxx.lu.