How do I attach lights to the top of wrought iron fence in back yard?

We used zip ties.

Hi. If you want to make this a permanent fixture and this is not an antique wrought iron fence, the fence it hollow. You can drill right into the fence and attach whatever you want. Good luck!

Black zip ties,baling wire spray paint wire black w/rustoleum to blend it to fence

Yep, zip, also called cable ties, come in different colours, lengths, and widths. They will need to be cut off with scissors, but are strong enough to hold lights securely in the wind.

Yes, zip ties are great! Easy to use and affordable.