To clarify things from the start, if you don't know or aren't sure about the distinction between "making fun of" and "humiliating" someone, you should seriously stop reading this. This article is definitely not for you.
Now that they're gone (hopefully) let's start talking about what trolling actually is. The official definition is: the action of making deliberately provocative comments on forums or other discussion groups in order to harass and trigger an emotional response from the other users.
However, in a broader sense, trolling can also mean to pull a prank on someone or sarcastically jest about a subject. It's a bit similar to the jokes that you make on April Fool's day only that you don't have to wait for a specific calendar date. As far as I'm concerned, trolling should always be fun for the troll as well as for the trolled person. It's a prank designed to lighten the mood and give both of you a good laugh without actually (physically or emotionally) harming "the victim". So, no humiliating and no bullying. And don't forget that what goes around comes back around.
How to harmlessly troll your friends with the help of your computers? I'll start you off with a few methods, but afterwards you're gonna have to come up with your own ideas. The most effortless way to troll someone is to create a fake newspaper article about the respective person. Doing so is easy if you visit websites like Shareonfb, Noob or Inthepaper. All you have to do is to fill out the respective forms, and your friend might have been sued for illegally downloading music, or who knows what other crazy stuff you can think of.
Another good way to have fun at your friend's expense is face swapping (or face dubbing). There are several applications that will enable you to easily copy a face and paste it over other ones. For example, you will be able to put your buddy's face over Shrek's body. Unfortunately, I couldn't find any good computer programs that can do this for free, but if you have some money to spend, you can try Foto Face Swap, FaceOnBody or Face Off Max. Android users can try the Face Swap app for free.
Furthermore, you can use specialized applications such as Fun Morph or Sqirlz Morph (both are free) to morph multiple faces into one. If you've ever wondered how a mix between you and another person would look like, these tools will give the perfect way to find out. Caricature or image warping are also nice solutions for trolling. Programs like FaceFun will allow you to deform and distort any photo you desire.
Want to trick your friend into believing you're a robot or an alien? That can be done easily with the help of free voice changing applications like Voxal Voice Changer and MorphVOX Junior. These tools will alter your voice when you use the in-game voice features or when you call friends on instant messaging applications.
Probably the best way to make your friends smile is to come up with the perfect meme. It is a funny image, video or even text that is rapidly spreading between Internet users. If you have some hilarious ideas, free computer programs like iMeme and Free Meme Creator will offer you everything you need to transform these ideas into viral memes.
Despite all the people who misuse it, trolling can be a smart way to further campaigns and cause massive reactions for a worthy cause. As an example, you can check out this video which was recently suggested to me by a mate (cheers!). It's about how a famous TV anchor decided to raise attention on the net neutrality topic.