Getting it right
The send-off is a piece of choreography
It is the only moment of the night where every guest does the same thing at the same time. It repays five minutes of planning.
Short answer
Hand them out before you gather the line. Light in waves from four or five starting points at once. Then have the couple wait until the whole line is lit before they start walking.
Hand them out first
Distribute before you ask anyone to form a line. A guest holding an unlit sparkler will walk to where they are told. A guest being handed one mid-formation will stop and block the line.
Give the job to two or three people and tell them beforehand. The alternative is your coordinator doing it alone while a hundred people wait.
Light in waves, not in sequence
One person lighting down a line of sixty will reach the end after the beginning has burned out. Start from four or five points along the line at once and let it spread outward from each.
Butane lighters beat matches, and a lit sparkler lights the next one faster than any lighter does.
Wait for the line
The couple should not start walking until the line is lit all the way down. Walking early means the far end is still fumbling while the photographer is already shooting.
Tell the photographer the plan
They will want to know where you start, which direction you walk, and whether you plan to stop in the middle. Say so if you are planning a kiss halfway down. That is the shot, and it cannot be taken twice.
Have a bucket at the end
A metal bucket of sand or water at the exit. Spent sparkler wires stay hot for a long time after the flame goes, and a bin liner is not where they should go.
Ask your venue what they require. Some specify it, and the ones that do have a reason.