This can be made in any length. Use 72" width tulle for shorter lengths than fingertip, 108" width tulle for fingertip and longer.
Begin by laying out and straightening the tulle. If you do not have a work table large enough, a clean floor is a good surface. If there are creases or wrinkles, they can be steamed out with a steamer or by hanging the tulle in a steamy bathroom. A regular steam iron is perfectly fine and will not melt the tulle as long as you use the appropriate synthetic setting; always test a scrap before pressing fabric.
Small weights along the edge are helpful to keep it in place and more easily repositioned than pins. Leave the center fold intact.
The bottom may be shaped into a curve using a compass technique. Remember high school geometry? Measure off a square at the bottom the same height as the width of the tulle, folded. Use a yardstick or tape to measure the same distance from this point and it will trace the curve. You can mark the edge with pins or use a dressmaker's marking pencil. Even a regular pencil will work if you then cut just within the line so no mark is left on the veil.

The veil edge may also be just rounded off freehand or traced using a platter or other round object. This will leave more width at the bottom of the veil, but will not create a ripple or cascade effect.

Use sharp dressmaking shears or a rotary cutter for a smooth edge. Open up the tulle and gather along the top edge.

A heavy button thread is good and won't break. A double strand of regular sewing thread will also work nicely. This can be done with a traditional running stitch, but it is better to use a whipstitch which encircles the edge. Pass the needle over and around the edge of the tulle, taking many stitches until the needle is filled, then draw the thread through. Magenta thread is used below for contrast.

It is easier to finish the veil edge before gathering. Attach to a comb, Velcro strip, or your headpiece.