Back when I used to tie my veil I would take both ends of the string and thread them down through opposite belt loops in my pants, then around my waist, up through the D-ring loop on the veil and back down to another set of opposite belt loops on the other side of my pants.
Different veils have tie-down strings and D-ring loops arranged in different ways, so giving generic directions is not too easy. If you must tie them on then I would recommend contacting those who supplied your particular veil for their directions in how to tie them.
About five years ago I stopped tying them on and just let them hang free. I wound up the tie cord on all of my veils into a compact knot (just in case I ever decide I want to start tying them again). I haven't, and after answering this thread I think I'll finally, cut the knots. I have been thinking of attaching a lightly weighted cloth tube to the bottom perimeter of the veils so that it rests gently on my shoulders, chest, and back, but I haven't bothered to do that yet, so a bee does sneak in every so often. Sometimes they get me with a sting, and sometimes I help them get back out safely.
Not having that cord tied onto my belt loops has made it so much easier to avoid accidents on the way to the bathroom -- I am one of those who suffer with I.B.S. Besides, I no longer have to wear pants that have belt loops, many of the shorts I wear now only have elastic waste bands or drawstrings.