Well, you plug them into your media player, put them on your head... Hah just kidding; I wouldn't recommend these for the gym. They would be too bulky and your ears would sweat so you'd be uncomfortable. The cord is way too long- you could use a rubber band or a cord clip to shorten it but it would still get in the way. Trust me, it's REALLLY long. Probably about six feet long- made for using in a bedroom setting listening to the TV. (The description probably says what the actual length is btw.) If you are willing to deal with an annoyingly long cord and sweaty ears while you are at the gym (presumably working out) then I guess these could work... unless you use the gym to, say... swim or do gymnastics. But why bother? You can buy cheaper, more practical headphones for the purpose of exercising.
And also- if you want can-style because you think they're better at noise blocking- these won't work for you either. My earbuds actually block out twice as much sound as these headphones do, so if that's your case- pass on these.
I would not wear them there because a) they're pretty big around your ears so you might look weird, b) they're over-ear so your ears would be very hot and c) they are pretty quiet headphones so if it is a loud gym you may not hear them so well.
Probably not well. They're not THAT tight on your head, especially if you're planning on running or even using an elliptical or just lifting. I would recommend these for deskwork, but earbuds for working out.
The cord is really long, but I was able to just tie excess cord into a bundle and keep it in a pocket. If you don't have pockets in your workout pants, or just want a more convenient pair of headphones, these may not be for you.