It's an acid. To really understand that think of how some acids eat through metal. Any TCA applied to the skin SHOULD be diluted! A lot! 100% TCA will burn through the skin on the face quickly, painfully, and with none too pleasant results.
If you must do it yourself (you really, really should not) do it slowly and start with very dilute amounts, like 10% (even 5%) for no more than a minute or two. If you like the results great, if not wait a couple weeks (after the week or so your skin needs to peel after the application) then try a little stronger (5% stronger) and another minute or two.
If you think you're all cool and these instructions are just for people who don't know better you'll learn. That's how you get bad results.
TCA peels are not for dealing with everyday mild acne. They are not a quick fix for anything. They are an actual medical procedure. Doing this yourself is pretty much as smart as doing dermabrasion yourself with sand paper or slicing open your face to do a face lift.
Better for those who insist on doing it themselves are glycolic or salicylic peels (still in small steps trying low dilutions and short times then working up, if needed).
Best is to just go to an actual dermatologist. You may not need to burn off a layer or two of your skin to improve it.

My boyfriend told me to try TCA Peel to get rid of my acne becasue he had a friend who had tried it and it had worked. Since I was desperate, I bought the TCA kit and followed the directions exactly. It did not help at all. In fact, it made it worse than it had been before. Don't waste your money. It is definitely not worth it.