How do i furnish bond or letter of undertaking for exports under gst? is there any format for this?

I am a freelancer providing web development services to overseas clients In GST model law you can avail tax benefit for export of supplies in two ways - 1) pay tax on the export of goods/services & claim refund of IGST or 2) export under Bond/Letter of Undertaking without payment of IGST and claim refund of ITC. I would like to know what is the procedure involved in furnishing Bond/Letter of Undertaking? And what is the format for the same? Many thanks in advance!