Get the Ring of Hircine as it will allow you to transform as often as you want as oppose to once a day. I got 9 feedings just on this quest - you have to agree to kill the hunters and not who Hircine sent you for. You have to equip the ring after each transformation because all of your gear is unequipped - making the ring a favorite will help. From there it is just a matter of killing and feeding until you get enough to earn the perks.
5 feeding - 1st perk
7 more feedings - 2nd perk
8 more feedings - 3rd perk
9 more feedings - 4th perk
12 more feedings - 5th perk
14 more feedings - 6th perk
16 more feedings - 7th perk
18 more feedings - 8th perk
20 more feedings - 9th perk
22 more feedings - 10th perk
24 more feedings - 11th perk
There are 11 perks so you will need to gain all of them which is 155 feedings.
To feed you need to get close to a corpse, you do not have to have been the one to kill them, and a prompt to press (A) will appear to feed - this will extend your Bloodlust and allow you to stay a werewolf for longer. The savage feeding perk allows you to feed on most dead creatures - making it easier to get the required kills - all the way right on the skill tree. There are a few things you cannot feed on: Vampires, Skeletons, Draugr, Spriggans and Dragons.
You can get to the skill tree by trying to open the (B) menu.
Some good places to feed are Stormcloak forts - I would suggest fast traveling first because the map is not available to you when you are a werewolf. Also, I've been told that northern Skyrim has lots of easy prey like wolves and such - thanks Malicious. I just ran from quest to quest as a werewolf hunting deer and other animals.
Another tip is that you can cast soul trap for an extra feeding - thanks Soto928.
As Party of Thr33 has pointed out the Dragonborn DLC has a few places that have numerous goblin like enemies call rieklings. Bristleback Cave may have the most of these enemies. You can find it south of Northshore Landing in the DLC.