tutorial.gst.gov.in

The GST portal's own FAQ on the Letter of Undertaking: who must furnish it before exporting goods or services without paying IGST, including services to a client outside India, and where to file it on the portal (Services > User Services > Furnish Letter of Undertaking).

FAQs: Filing of Letter of Undertaking for export of goods or services (Form GST RFD-11)

From tutorial.gst.gov.in by GSTN FAQ page

• File the LUT in Form RFD-11 on the GST portal before you bill a foreign client without IGST.
• It covers exports of services as well as goods, and supplies to SEZs.
• Select the financial year it applies to, so file it again each year.