Step 1: The basic concept and materials required.
A kite arch is a string of several tens of kites, all side by side. Instead of each flying from a line fixed to the front, they all fly "shoulder to shoulder" along a line fixed across where the horizontal spar usually goes on a lone kite.
You will need:
A heart template: Download the Word document I have attached (it will need to be copied up to A3), or find your own outline that you like.
Kite line: Something around 100lb test line should be fine. It doesn't need to be pre-stretched, and monofilament fishing line would probably work.
Sail material: Paper or mylar wrapping film will be easiest, unless you want to use the arch again next year, in which case use tyvek if you can get it.
Adhesive tape: If you're using paper, mylar or tyvek for the kites ordinary sticky tape is fine. If you're using something exotic, make sure your tape will stick to it.
Bamboo spars: Cheap barbeque skewers are ideal, the kind that you buy in packs of 100 for a pound from the supermarket. Otherwise, I've had great success with the matchstick-thickness bamboo from roller-blinds.
Tails: About 2 feet per kite, light ribbon, strips of plastic, anything that is light, flexible and complements your kites would be ideal.