We must use very strong wire 7-8 gauge (3-3,5 mm) for hair pins. Because they have to withstand the load. But working with such a thick wire is difficult. But if you heat the copper, it becomes soft and flexible. And this makes the life of wire worker much easier.
Use a very large hammer to make it flat.
Use butane torch to heat the wire to the red light.
Repeat again to beat with a hammer and heat.
After the heating (until the wire is soft), use a bench vise and big pliers.
Hold the wire and twist it.
The ends of the wire are very scratched now. But the hairpins should be smooth.
Use files to make them smooth from rough to small.
And use Rubber Polishing Wheel for making it ideal!
Then you can solder any parts to the top of the hairpin. Clear in the solution of citric acid. And I beat it by a rubber hammer to turn it back to strength after heating.