This is the technical part that HardReset.info details clearly. You need to press a specific combination of buttons:
On Android 7.0+, this feature is mostly removed. How to Unlock ZTE Blade A520 - HardReset.info
If your ZTE Blade A520 is still on Android 5.0–6.0 (unlikely but possible): This is the technical part that HardReset
By following these steps, your ZTE Blade A520 will be restored to its original factory state, effectively bypassing any software locks or performance glitches. How to Unlock ZTE Blade A520 - HardReset.info
On the next screen, you will see a list of "No" options and one "Yes" option. Scroll down to "Yes -- delete all user data" and press the Power button.