Instructions
Turn you Canon T2i and press the "W/B" button and turn the dial until you are at the "Custom W/B" setting which looks like two directional arrows pointing towards the middle.
Place the grey card in front of the camera lens in the lighting conditions you will be shooting in. When setting a custom white balance you need to set it under the lights you will shooting in, if you set the white balance under one type of light, then change the lights the white balance will no longer be useful.
Take a picture of the grey card. Once the picture is taken, press the menu button on the back of the camera and toggle to the "Set White Balance." Select this field and the picture of the grey card you just shot will appear on the screen. Press the "Menu" button to set the camera's white balance to this grey card.