In FRONT of your cables
I can't tell you how many times we've had to call out "What's your number," to riders who've installed their numbers behind the cables, making it difficult for the finish line staff to read the numbers. More than once riders have kept going and we never did get a number, so those riders didn't get a finish time.
Do yourself and the event's staff a favor and make sure that your number is in FRONT of your cables/brake lines/etc. so that the number can be easily read and you don't get missed.
And please, don't fold it back around head tube to make it "more aero" as some like to do. I definitely can't be read that way.