How to book a tatkal ticket from IRCTC.

Please note : this is based on recent personal experiences trying to book train tickets via the IRCTC portal.

So you have an unplanned travel ahead and cannot / won't shell out money for expensive flight tickets. Tatkal train tickets is the option for you, but if you're trying to get lucky with tatkal tickets, the probably and chance game is like that of winning a contest. So all the best!

1. The route: Chances of scoring tatkal tickets on a popular route eg. Mumbai to Delhi is much better than getting it for an unpopular route for instance Panvel to Gwalior. Because less trains / less frequent trains hence more competition for you.
2. Internet Connection: Your Lan / broadband / wifi/ Hotspot better be strong enough to survive the way till the end. If your Google.com takes more than a couple of seconds to load, you will fail.
3. Typing skills: now it's best to have a master passenger list saved in advance, but u still have to type a few things like to and from stations, enter dates etc.. Payment card details and more. So u better do ur typing test and be good at operating/navigating the laptop/computer!
4. 10am alarm! Now even if u login before, u may have to relogin again and the server becomes really really slow. I almost feel that it's a conspiracy where you won't be able to login because no captcha image or login error or server not responding (all of this happened with me). So be ready to bang your head with the keyboard in sheer frustration. (Timing for non-AC tickets is different. 11am)
5. No surety:  By God's grace and some luck if you enter all details and find availability in the train of your choice, there are good chances that by the time you make the payment what you've booked are tqwl tickets or tatkal waiting list tickets, the chances of confirmation of which is really really slim. As the general quota waiting list is given preference first. So basically, full popat. 
6. Why the app? There is an IRCTC mobile app that works faster, but once you enter details it sends you to log-in to your desktop-like IRCTC account (from the phone). What logic?
7. Success! Maybe after trying for 2-3 days you will finally hit jackpot and get confirmed tatkal tickets. And if you can have a fundraiser organized for your travel, you could book premium tatkal tickets. Still cheaper than last minute flights but way too expensive for train travel (double the cost of a tatkal Tkt and equal to flight if you had booked it months in advance!)

So, if you think you may travel later, even if there is a remote hunch - I would suggest you to book tickets and bear the nominal cancelation charges if you don't go. It's cheaper than the tatkal surcharge and fares. And I get sweaty palms and a racing heartbeat every time I try to book tatkal tickets - the pressure and anxiety of getting one for the family members I tell you! So if it's a life and death situation, I suggest you go to a known agent.