There will probably not be enough material in the halter straps to do the conversion.

1 - Find material that blends with the top.

Would widening the strap help? If so, attach wider material under the existing straps.

If wider straps don't help:

2 - Make a test strap that is long enough to run from the nipple area to the back strap, and wide enough to be comfortable ... you need to test various positions,

3 - Test various positions for attaching the new strap ... about halfway between your collar bone and the top of your breast is a good starting place.

4 - When you know where the best place is, make the straps out of your new material and sew them on.

To make this look planned, use that material as trim elsewhere on the swim suit.

The first step would be to see if the halter hurts because the strap around your neck isn't long enough for you, putting excess pressure on your neck. Simply lengthening that strap may be enough to make it comfortable.

If not, since the halter is designed to curve around the neck, it might work best to go for a criss cross in the back. Lazygardens is right...you won't have enough fabric so you will have to buy more in something coordinating.

If the fabric is bathing suit fabric (2 way stretch) you will have to know how to sew on it using elastic and multiple-stitch zigzag.

If not, then you should just be able to attach the new fabric by seaming the new fabric to the old in the shoulder area and then attaching it to the bra band in the back.

Thank you for your suggestions. The tops both have plenty of length they tie around the neck. The straps are are about 1 inch wide material. I was thinking of putting buttons or hooks on bra band and somehow attaching them that way.

They will be more secure if you make them the right length and sew them on.

If you have plenty of length, get a helper and have the helper pin them to the back band in various positions.

You lift the bra cups up from the top until it's comfortable and hold the cups there while the helper pins the straps snugly. Try it, and if it's OK, stitch it.

I had a couple of button-strap tops and the button popped.