Introduction
The Shield of Arrav quest requires two players to work together in order to complete the quest. One player has to be part of the Phoenix Gang and the other has to be in the Black Arm Gang. If you don't have any friends available to help you, look on THIS THREAD on the Official RuneScape Forums.
Not in the Phoenix Gang? Click for the Black Arm Gang!
Phoenix Gang
Starting Out
Speak to Reldo who is located in the Varrock Castle Library and ask him for a quest. He will tell you to look in a book titled "The Shield of Arrav" for a quest. Look at the bookcases carefully and pick out the one with the dark blue book. 
Now read or skim through the book. Speak to Reldo again to find out where the gang is located. He isn't exactly sure but he thinks that Baraek the fur trader, located right outside the castle walls, might know.
Speak to Baraek and ask him if he knows the location of the Phoenix Gang. He will require you pay him 20 gold coins for this. He will then tell you the location of the hideout.
Phoenix Hideout
Go speak to Straven who is down the ladder guarding the door into the hideout. He says that if you want to join you will need to prove that you are willing to work for the gang. There is a member from the opposing gang that has some information that the gang wants in an intel report. Go kill Johnny the Beard at the Blue Moon Inn to retrieve this report. Take the report back to Straven and he will allow you to join and will give you a key to their weapons storage and access to the hideout. Now go to south-west room in the hideout and search the chest. You will find one half of the Shield of Arrav. 
Finishing Up
Give the Weapon storage key to your partner so they can get the crossbows, then wait for them to get their half of the shield and go with your partner to King Roald in the Varrock Castle. He will tell you to get it authenticated from the Varrock Museum. "Use" the shield on Curator Haig Halen. After your partner has given the other half to the Curator, trade your partner, and both put up your certificate halves.
Use one certificate on the other to get the whole certificate, and go back to the King to claim your reward.
Black Arm Gang
Starting
Talk to Charlie the Tramp near the Varrock Sword Shop. Ask what's down the alley, and he will tell you about the Black Arm Gang. Ask if he thinks they'd let you join, he'll tell you go in and talk to Katrine. Talk to her, and say that you've heard they're the Black Arm Gang. Tell her your source is Charlie the Tramp. Tell her you want to become a member of their gang. Ask her if she can't give you a try, and she'll finally a agree on one condition.
Crossbows
She'll ask you to get two crossbows from their rival's - the Phoenix Gang's - weapons stash. You'll ask why two, and she tells you her plan to frame the Phoenix gang. Agree to get them, and she'll tell you it's next to the Zamorak Temple in south-east Varrock. If you go there, you'll get a message saying you'll need help from another adventurer.
Have your partner trade you the Weapon store key, go in, and kill the Weapons master. in there, and take the crossbows. Go back to Katrine and give them to her.
Shield and Reward
Enter the door on the north side of the room and head up the stairs. Open the cubbard south of the stairs on the wall, then right-click search to find the shield. Exit the gang's building. Go with your partner to King Roald in the Varrock Castle. He will tell you to get it authenticated from the Varrock Museum. "Use" the shield on Curator Haig Halen. After your partner has given the other half to the Curator, trade your partner, and both put up your certificate halves.
Use one certificate on the other to get the whole certificate, and go back to the King to claim your reward.
