Chro-me?

1. Place the trap in front of the portal that the Chromie is spawning at
2. It takes a second or two for the trap to arm, it's ready when the hourglass appears. The trap lasts until cancelled
3. When the Chromie runs over it they will be trapped

It can take a couple seconds to get credit for it, don't panic!

This is the fourth quest of eight, in an optional quest chain you can do, that will lead you to the new Mythic dungeon called Dawn of the Infinate. By optional, I mean you don't actually have to do it to start running Mythics inside it. However, if you would like a little bit of lore, you can begin this quest chain by flying to the top of the Seat of the Aspects, in Valdrakken, and speak to Nozdormu [/way #2112 61.0 39.6 Nozdormu].

When doing this specific quest, as another has stated, you can do this by placing the trap in front of the portal. However, This does NOT always catch Chromie the first go, as it takes a second for the trap to actually activate after being placed. If you happen to place it a second before she runs through it, she will just pass right through it causing you to have to wait longer for Chromie to re-appear so she can then run through the trap and get caught. It will work better if you try and give yourself a few more seconds between her and the trap.

While it IS far easier to do this by placing the trap in front of the portal, you don't have to do it there all the time. You just need to have it up so Chromie will run through it. It's also possible to have multiple traps up at a single time.

EDIT: This comment originally also referenced the list of story quests because the wowhead pages, for each quest, did not have them listed. They do now and thus the individual quest chain IDs listed in this post are now no longer needed and have been removed.

For those having a hard time timing the trap, be aware that the Time Trap isn't active until the hourglass appears. The whole sparkly light show for the first second and a half is completely functionless.

That said, the Trap stays out for a very long time, and the various Chromie's paths never change. As long as you drop it somewhere the Chromie has walked, it's guaranteed to catch her by the next loop.