Right! I had a picture initially, but there just wasn’t enough room for it.
In the picture, it is just pinned to Josh’s shirt, but how you attach it depends on what character you’re using it for. For Mon-El, it will attach via snaps placed over the collarbones, with decorative discs overtop.
I wholeheartedly recommend snaps as the number one way of attaching capes –– I cannot stress enough how dangerous it can be to have capes sewn directly to your shirt or clasped at your throat, simply because if someone steps on your cape or it gets caught in something, the cape releases instead of yanking you by your neck or trapping you. Of course, some cape styles don’t allow for this –– Supergirl’s Turner cape, for example, loses a lot aesthetically when you attach it via snaps — but whenever possible, please consider your own safety and comfort and just use snaps. Big snaps are certainly sturdy enough to hold it in place, and it also makes it easier to take apart to store, iron, wash, etceteras. Don’t underestimate the usefulness of being able to remove it easily, it just makes your life as a cosplayer a million times easier.
Velcro is another option, but I don’t really recommend it. I hate velcro. I hate the sound, I hate the texture, I hate the finicking to get it lined up properly when you put it on. I hate it. But most heinous of all, velcro and spandex tend to be destructive when put together, as velcro can easily tack to the surface of spandex and destroy the surface when you try to take it off. If you must use velcro, use a lightweight kind.
Alternately, you can close it at your neck with a clasp, but again, I don’t like things that join around the neck. I just cringe whenever I see people wear capes that clasp there, and we altered Emmy’s Batgirl cape so that it would attach from the shoulders instead of the neck for that very reason. I just imagine it getting caught in a door or on an escalator or having someone step on it and cringe. It’s also a matter of weight — if your cape is heavy, then you have all that pressure put directly on your neck all day… that can hurt. So please, please, think of your body when you cosplay and make it easier on yourself.
If you must, though, sew snaps to the back of the neck so that the cape is more supported by your shoulders instead of your neck. This also helps in that if someone steps on your cape, you’ll feel it on the nape of your neck before you feel it on your throat.