The Internet allows us to form new identities and express controversial ideas without fear of personal reprisal. It allows us to explore interests that would be misunderstood by friends and family and discuss experiences that may damage our career.
There are many reasons to want to create a pseudonymous identity online. You may have undergone a traumatic experience you don't feel comfortable talking about in public. You may be the head of a stuffy art museum, but love writing Doctor Who fan-fiction. You may want to write a scandalous tell-all blog. Or you may be a rabid democrat living in an extremely-conservative small town. You may just find it liberating to be freed from the biases society holds against your offline life or background.
How to Browse Without Being Tracked
The first step is to begin using Tor. Tor is a free browser that obscures your IP address by using onion proxy software that provides multi-layer encryption. While not tied to you, personally, your IP address is associated with your individual computer, hence a skilled individual may use your IP address to discover your offline identity. This is why it is important to not rely on services like Chrome's incognito mode to attempt to live your double life online; while it will prevent websites from storing things on your computer, it will not hide your IP address.
To download the Tor Browser Bundle, which includes everything you need to browse the net anonymously, visit Tor's website and follow their instructions (also read up on using Tor properly- there are some things you can do using Tor that might still reveal your identity if you are not careful). You should use Tor Browser for all of your secret online activities. It may run a bit slower (as your interactions with other sites are bounced through at least three relays), but it is better to be safe and slow than speedy and sorry.
Use Tor for regular activities as well. This mitigates risk associated with others noticing your browser choice. When questioned about Tor, simply say you use Tor Browser because of concerns about privacy, which have been exacerbated because of this year's revelations about the NSA.
How to Lay the Foundation of Your Double Life
Most online accounts require an email address. It should go without saying that you should not be using your personal email address when doing anything related to your pseudonymous identity. Instead, you should create a separate email that has no connection to your normal identity and is only used in connection to your covert activities.
I recommend using a free service such as Hushmail, Gmail, or Riseup.net to create this account rather than Yahoo or Hotmail, as the latter options include the IP address of the computer used to send a particular message. While this matters less when you are using Tor, you may still find yourself in a situation in which you need to check that account without the browser's protection, hence more secretive email providers are better.
Tips on Developing Sound and Secretive Habits
With Tor Browser and a designated email account, you are free to live out your double life- so long as you do so carefully.
Never ever use a work computer for secretive activities. I don't care how encrypted your work communications are. Company-owned computers are not to be trusted. Keylogging software, which will make all of your careful precautions amount to nothing, is only one of many potential complications.
Avoid using mobile devices. They can be lost. They can be stolen. Strangers (or worse yet, friends, family, and colleagues) can more easily look over your shoulder or snatch your device out of your hands. If you must conduct some of your double life through a mobile device, make sure it is password protected, only use it in private spaces, and bolster it with additional privacy protections, which, for the sake of brevity, I recommend you find independently.
If your double life involves posting content (e.g. blog posts about your secret ventures as an undercover nun), schedule your posts (many blogging platforms offer this functionality) so that they are published at random intervals that cannot be associated with a specific time zone or lifestyle. Do not tag photos, posts, or tweets with your location.
Important Identifying Information to Hide
Obvious information that might be used to determine who you are (eg. your name or the names of people in your life, your personal email address, identifying photos. etc.) is but a small factor to consider. Most secret identities are discovered by those who use more subtle hints to piece together your personal puzzle.
Don't give away hints by even letting your pseudonym resemble your real name. If your name is John Doe, your online handle should be entirely different, like Shane Kugel (and not J.D. or Shawn Moe). Be extremely careful about mentioning employers in a manner that would enable the casual viewer to narrow your real employer down to a couple of candidates (e.g. "I work for a pet grooming salon in San Francisco, California"). Also be mindful with regard to any habits, sayings, or possessions you might mention (e.g. a stormtrooper figurine kept at your desk) that could be identified by those who know you in real life.
If you maintain a website, make your WHOIS information private. If you do not, everything from your name to your email, phone number, and address may show up in WHOIS queries (just search for your friends' domains to get an idea of the information that might be revealed).
Should you be involved in commerce, opt for trades whenever possible. Gift cards might be a convenient form of currency, so long as you keep the value of transactions below $500. Generally speaking, money is difficult to keep anonymous online- even when Bitcoin is used.
The Importance Leaving No Trace
Whenever you finish a session of secretive internet activity, your computer should be devoid of damning information. While it helps that you are using Tor, it is also important that you delete any files from your computer related to your pseudonymous identity (e.g. drafts of blog posts, photos, etc.) before you get up and walk away.
You never know who might poke around your desktop when you aren't looking, and you would be surprised by how many friends and family members know the passwords to their loved ones' machines.
This brief guide is an introduction, not a comprehensive playbook. Its recommended tactics will help you avoid major mistakes and may accommodate "harmless" double lives, but if you are involved in some serious whistleblowing activity, fighting against a totalitarian government, or are threatening to take down a beloved member of 4Chan, you're playing an entirely different ballgame.