If it is going over a bikini or a bodysuit: a body chain bra. It is built to sit on top of something and follow the same line. That is the whole design, and it is why it works on a blank outfit better than on a busy one.
On fit: measure the underbust, ignore the size label. Chain does not stretch, so the number that matters is the one around your ribs. Most adjust through an extender at the back, and that extender is what makes or breaks it.
If your skin reacts to metal: jewelry goes on last and comes off first. Lotion, perfume and sweat are what break plating down, not wearing it. On last after everything has dried, off before a shower, and dry before it goes back in the box.
If it is a gift: custom engraving, and order it early. Anything engraved to order takes longer than a stocked piece, and the whole point is the words, which are worth getting right rather than fast.
If you want the joke to land: a bar for every day, a heart for a present. A bar reads as a plain name necklace until somebody is close enough to read it. A heart announces itself, which is what you want when it is being unwrapped in front of people.
If it is for photographs: rhinestone and crystal over plain metal. Set stones catch light from every angle. A plain chain reads as a thin line at any distance, which is the right choice in person and the wrong one on camera.
On building the rest of it: one hard piece, not three. A chain bra, a harness and a collar all at once fight each other. Pick the one doing the most work and keep the rest quiet. See accessories for what else finishes it.