Before you start check that you have the parts you need: an assembled PiTFT 2.4" HAT with the 2.4" screen and 2 x 20 female header. Note that it is normal for the screen to be 'loose' - this is so its easier for you to solder the connector on!
Check also on the back that the TFT is attached and that the flex connector is seated into the onboard FPC socket.
The easiest way to attach the header is if you have a Raspberry Pi as a 'stand' - make sure its powered off & unplugged!
Plug the extra tall female header into the GPIO port on the Pi as shown. Make sure its seated nice and flat
Place the PiTFT HAT on top so all the pins stick through the connector on the side. Gently flip the TFT so its off to the side and wont be in your way while you solder
Heat up your soldering iron, and grab some solder. Start by tack-soldering one of the corners. Then you can re-heat that solder point and adjust the circuit board to make it sit flat.
Before attaching the display, check that all the pins are soldered nicely and there's no bridging, cold solder, shorts, or unsoldered pins.
Now we can attach the screen. Remove the two thin tape cover strips.
You may find that some extra double-sided tape will keep the screen in place better than the two little tape strips! Put a piece or two on the back.
Line up the TFT screen so it matches the white outline and press it down to stick it to the circuit board