The big thing I always suggest is to remember that you are not trying to show that your child is "merely" gifted, you're trying to show that your child is beyond the range of testing instruments. Be selective. Don't include cute stuff or things that are 1-2 grade levels above the norm. Include things that most persuasively demonstrate that your child is HG+.

Video of your child in action can be a good idea.

Best wishes!

Kriston