Putting the back panel on your device works in the opposite way as you did taking the back panel off. Start by pressing in the bottom sides of the back panel onto the phone and continue up the phone until you have completely attached the back panel onto the phone.
Note: If you used a replacement guide that required you to take the power button off, make sure you place the power button on BEFORE adding the motherboard (following the guide backwards). Not doing so will result in you not being able to fully close the back panel due to the power button not being placed where it needs to be.