Once you’ve completed the Eagles’ Peak quest in Runescape, you can start trapping ferrets in the Woodland hunting area near the Piscatoris Fishing Colony. As a reward for that quest, Nickolaus will teach you how to trap ferrets, and he’ll even give you a box trap so you can start right away.
You can purchase additional box traps at the hunter shops in Yanille and Nardah for 38 gp each. It’s also possible to purchase them at the Grand Exchange in Varrock, but expect to pay a higher price for them if you decide to go that route. While you’re buying supplies, pick up a few rabbit snares. You’ll need them along with a ferret to catch rabbits, and they only cost 18 gp at the hunter shops.
Ferrets can be found near Nickolaus’ tent on the northwest side of Eagles’ Peak. The screenshot below shows the exact location. (Click any image for a larger view.)

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Walk out to the middle of the area where the ferrets are running around and click on the box trap in the inventory to lay the trap.

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All you have to do now is wait. If you want to increase your chances of catching a ferret right away, you can bait the trap with any type of raw meat or, if you are Hunter level 39 or higher, you can smoke the trap. However, neither one of these things is necessary so I usually don’t even bother with them.
Once a ferret has triggered the trap, the box will close up like the one shown in the screenshot below. It is possible for a ferret to trigger the trap without getting caught in it, so you may have to dismantle the device and start over.

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When you do manage to trap a ferret (and it shouldn’t take that long at all), you can retrieve it from the box trap and it will appear in your inventory. Now, you can move on to hunting rabbits.