Power off the device by holding the power button until the "Power Off" popup appears. Tap the popup and wait for the device to turn off.
Insert a SIM card eject tool or a paper clip into the hole next to the SIM card slot, which is located just under the power button. Do not force the paper clip if it is too thick; instead use the head of a sewing needle.
Push the tool into the hole until you see the card pop out.
Use a plastic opening tool to free the clips on the top and each side of the back cover.
The bottom is secured with some adhesive foam. You should be able to finally free the back cover using an opening pick. The "snaps" at the bottom edge are more like hinges, so release all others then swing the back cover off outward from the top, and these fasteners will release without breaking.
It's best to avoid prying around the SIM card slot and volume controls, as the plastic in these areas is very thin and likely to break from prying.
When you replace the cover, there is a snap about even with the flash window that must also be attached (one of three that secure the back cover to the midframe). Qi (wireless) charging may not work if this isn't secure. Apply gentle but firm pressure over the "S" in "Nexus" and slide your thumb up towards the top, and the snap will engage.
Remove the six 4.0 mm Phillips #00 screws that secure the midframe to the phone.
Ensure you retain all six of the 4.0mm screws! These hold the midframe securely and closely to the motherboard. The three small snaps on the back cover that attach to the midframe ensure the NFC antenna and wireless charging coil connectors contact securely with the motherboard, and work reliably.
To replace the LCD screen and digitizer, remove the additional four 4.0mm daughterboard cover screws with the same screwdriver. You may have to pierce through clear adhesive material to remove them.
Examine the midframe and the back cover and you'll find three very small snap attachment points for the back to the midframe. These ensure that the NFC and wireless charging antenna contacts connect with the corresponding spring connectors on the motherboard. Ensure the snaps are in good condition.