If you are purchasing individual tickets for TIFF 2012, you may first get shunted to a virtual waiting room if the load on the system is high.
Note that the countdown timer refers to when the page will automatically refresh, not necessarily when you will be let in; the counter may simply reset to a new number (and that number is pretty random, on refresh I've seen new times anywhere from a few hundred seconds to over 2,000). You may also want to try opening multiple browser windows or tabs as well; I was let in on one that wasn't the very first one I opened.
Once you are let into the system, you will see a welcome page:
You can then select films by date (By Schedule) or by title. The image below shows picking by date:
When you click a date in the calendar, you will see a list of all screenings. Click on the time of the screening you want to attend:
You will then be taken to a screen where you can indicate how many tickets (up to 4 per screening) you wish to purchase:
The tickets will be placed in your shopping cart. Note the timer at the top of the screen. Once you pick your first screening, you have 30 minutes to check out. I'd recommend you leave time to actually complete the payment process within that time limit.
If you want to pick by title, you'll get a screen where you can click a letter of the alphabet and see all films starting with that letter. Note that in the My Choice ticketing process, people had more luck (i.e. less errors) using the By Schedule view.
If you click a title, at the bottom of the page you will see all the screenings for that film. Click the one you want to see, and then you will be directed to the quantity page as shown previously.
When you are ready to checkout, click the Checkout button at the top of the screen. Fill out the relevant information and finish the payment process:
If you encounter a server error any time during the process, it appears that some people have had success refreshing their browser; your shopping cart may need to be refilled after doing this however. Note, do this at your own risk as it's also possible you could get booted back to the virtual waiting room if past experience is any guide. You could also try calling TIFF customer service, but it's unlikely you'll be able to get through to anyone as the phone lines will likely be busy with people trying to phone in their orders.
I ran into an error once while I was online, and I had to refresh about 3 times before it took me back into the site, but other than that there were no problems.
If a screening is marked off-sale, it is sold out. However, some people have reported checking back on the film before they checked out of the site, and a previously off-sale film had appeared back on-sale, so you may want to double-check.