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.

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

1) Obtain the Ring of Hircine from the side quest "I'll be met by the moonlight" which starts in Falkreath and ends with you saving a fellow werewolf named Sinding from a group of hunters. Once you have the ring, you can use it to transform without limitations.
2) Feed at places with regenerating humanoids. Personally, I love to feed on the werewolf hunters themselves at the Silver Hand camp/fort at Gallows Rock, which is located southwest of Windhelm.
3) Unlock the Savage Feeding Perk as soon as possible to earn the ability to feed on animals and other living creatures around the world for slightly reduced gained health and time from them.
4) Become strong enough and complete your fights as a werewolf and eat as you kill, using a howl while feeding will still give you the perks of feeding and cancel the action to save time and health from nearby attackers' damaging attacks.
5) Complete the Companions questline in order to be able to talk to Aela the Huntress about receiving each of the three Totems of Hircine and work to find the Totem of the Hunt, then upgrade it for maximum humanoid and victim detection.