Talk to Hawkfeather to get the task, then talk to Beestripe to learn information, then talk with Hawkfeather again, then talk to the light gray she-cat at the farm, then lastly, talk to Hawkfeather again.
If you complete the first half of the achievement "Nightmares" (Going to Hawkfeather, Talking to Hawkfeather, Going to Beestripe, Talking to Beestripe for a long time) then come back to Hawkfeather, she'll point the way. Go that way and click the new arrow that pops up.
