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.
Step 2: Make the Individual Kites.
If the material is not decorative, now is the time to decorate them with paints or marker pens. Let them dry.
(A thought occurs to me: you may want to add a message to the arch, one letter per kite. This is a Valentine Instructable, so make it romantic.)
Stick two bamboo skewers to the back of the sail, from the lower point of the heart to the top of each upper curve (the skewers will make a narrow 'V'). Tape is the easiest way to do it, but PVA glue will stick bamboo to paper just as well. If you use PVA, allow drying time again.
Staple the tail to the kite, at the point of the heart. If you are using very light tails (say, strips of plastic bag or "STOP POLICE" tape), then you may want to attach two or three per kite.
Stack your kites carefully, ready for the next stage.
Step 3: Construct the Arch.
Leave about 10 feet of line, then lay it across the back of the first kite at the widest point. Tape it in place. Leaving about a foot between each kite, tape them all securely to the line until you run out of kites or line. You should leave another ten feet (plus loop) at the other end as well.
Stack the kites carefully as you attach them, or they will never fly.
Step 4: Launching.
Find a clear space and stake down one end of your arch (tent pegs or dog-stakes are good for this, alternatively tie the line to a heavy piece of the scenery).
Walk the arch downwind, unfolding as you go. Keep the arch as clear of the ground as possible.
When you get to the end of the arch, turn and walk across the wind, hold the arch up as much as possible. As the wind catches the kites, they will lift.
When the arch is flying at the height you desire, stake down the second end and enjoy the spectacle.