Shortly after reaching Velen, you will find yourself seeking a village witch. When meeting her, you will find a mob of peasants begging for help, then follow her into her home... to see that she's missing. Investigate the home, and once you have, a portal will appear. Enter to find a mystical room and a witch naked in the bath.

Easy enough. Once you've completed the Keira quest, accept her ask for help when you next visit her hut. This will take you on a big spoiler alert adventure that isn't relevant. Complete it and let her know... and then do her ANOTHER favor, she's gonna ask!!

You've done the story quest, and the two revolving around Keira? Well, maybe she thinks you deserve a reward... and maybe not, but who knows. Either way, once you've retrieved a certain package, she claims to have something for you. Accept it, and it will go from there into a rather lovely evening. Sorry if my guide seems vague, I simply avoided as many spoilers as I could.