It happens better on slow computers...it'll just happen on it's own. You always look like the CT Navy Seal. When I did it for my first time by accident, I vowed to figure out how to do it. I did, this is my method:
When you're on the team selection menu, press Tab, then 1, then Tab, then 1 etc. pretty fast. Sometimes it's better to press Tab a few times then 1, then Tab a few times again, then 1 etc. If you do it correctly, you'll be at the skin selection menu. Do the same thing here. If you do it correctly, you should be back at the team selection menu. Pick CT, you'll stay at the menu, then click cancel. I say Tab and 1 because 1 is the shortcut for Ts, and you'll look like a CT. Before you start this, make sure there's at least 2 people dead. If not, you'll join a team and you won't be able to do the glitch.
It feels harder now after the recent CS updates...