Where is the instruction saying how and when to wear jump wings?

It depends on the service branch - each is a bit different on the qualifications for the Parachutist Badge, the formal name for Jump Wings. However, in most cases if the member has met the requirements for the badge and that qualification has been so entered into the member's service record, the member is entitled to wear the badge with the appropriate uniform.

Uniform Regulations state what military insignia must be worn and how, and with what uniform. For example, you'd wear ribbons and medals on a dress uniform for a formal ceremony, such as a Change of Command, but you wouldn't wear such things on a working uniform.

Warfare and other badges are a bit different though, and it's dependent on the individual service branch as I said. You need to look at the Uniform Regulations for the uniform in question, the service branch involved, and see what insignia is authorized to be worn with that uniform.

For example, as a Submariner I was (and still am) entitled to wear my Dolphins on my uniform. I could also wear them as a cloth patch insignia on my working uniform shirt.

It's also up to the individual's unit Command regulations as to how and when certain insignia are worn. If service regulations do not specifically prohibit wearing something, an individual command can authorize it. So just because it may not be authorized by service regulations doesn't necessarily mean it hasn't been authorized by a unit commander.