Please note: This quest is a massive waste of time and orens. If you have yet to complete the quest I recommend you skip the quest. If, however (like me), you despise the foul beasts then please continue.
Firstly you will need harpy traps. Seven in fact .These can be bought from the dwarf blacksmith in the north east of Vergen. His name is Haggard. Alternatively you may make your own using a diagram (also sold by the smith). Pre-made traps are 100 orens a shot so it will set you back a cool 700 orens. If you buy the diagram and make your own you will likely save yourself a few orens, though merely enough to justify the reward. Also you will need both 'The Harpy Contract' and 'The Harpy Queen Contract' or at least access to the Harpy Quarry.
Once you have your traps venture down to the quarry area by exiting Vergen through the western gate. Branch right and pass through the burned village. Make a left when you reach a small unit of Sco'aitel and Dwarves. Continue down the path. Just before you reach the burned house make sure you are hunting at night. If it's the middle of the day then meditate to around 23 (This would also be a good time to consume any potions if you so wish). Also make sure you have 'The Harpy Contract' as your tracked quest. Then you should see something like, 'Harpy Nests Destroyed 0/7'. This will allow you to see if your trap has worked.
Setting Traps: Continue down the path towards the quarry in search of harpies. Every time you get attacked by harpies lay down a trap. Block or use Quen sign and wait to see if a harpy picks it up. If they do the tally on the right hand of your screen should go up 1. If they don't pick up the trap it's because you're not near enough to a nest. If this happens just kill the harpies, pick up your trap (very important) and continue. I have found if you can see a nest and you place a trap in the area, it will not necessarily be that nest that's destroyed. So don't worry, if the tally increases it worked and move on. You should be able to get 4/7 outside and 3/7 inside. Don't worry if harpies aren't spawning it either means you're not hunting at the right time or the nearby nest has been destroyed.
Once you have 7/7 destroyed return to Cecil Burdon for your insulting reward. Due to the game not letting you read what you receive as a quest reward by overriding the text with the quest completion notification, I am unable to tell you exactly what you receive. Though I can say you get roughly 200 orens and nothing exciting.
I hope this helps you either complete the quest or skip it entirely (which is what you should do!)
Feel free to pass this on to any who made need the help.
Yes this quest is a pain in the behind, but at least it's kinda on the way to complete Hunting Magic or whatever the name of the main quest it is, so you can end up doing all those quests at the same time.
Oh BTW, I think you can get a harpy trap diagram if you buy and read the harpy book in Flotsam.
Yeah, what you wrote is truth, important thing is to realise that u must hunt down nests inside the harpy lair, u can do it during the quest where u are looking for a magic source, I just want to add that the reward for completing quest is: 200 orens, dwarven upgrade for armor, and fire rune upgrade for sword, but ye it is not worthy of it, I spent at least 1000 orens to get the traps and I dont remember getting any xp so you should skip this quest unless you want to have em all like me :D