[Finless]

Take it down your nearest dive shop/send it back to the manufacturer and get them to do it - at least you can moan at someone if it goes wrong.

Well - it is an option.

When I sent mine back to the suppliers for neck and cuff seals they gave the whole suit a once over and found a problem - I was becoming delaminated around the holly hocks. They sold me a brand new suit for not much more than the original repair + pocket and "neck warmer" I had requested. Boy, was I glad I'd sent my suit back.

I have seen them being done and one essential requirement is something to shove up through the neck hole (like a ball) to keep the material smooth and even and allow pressure to be applied to the new join.

Have fun.