Do this part while you still have strong WiFi at home, not at the gate. After you buy your MegaEsim plan you get a QR code by email. Open your phone settings, go to the mobile or cellular menu, choose to add an eSIM, and scan that code. The plan downloads in about a minute and then sits quietly on your phone.
Here is the important part. Installing is not the same as starting. The data does not begin counting yet. The plan only switches on when your phone first connects to Airtel in Zambia. So you can install today and fly next week with nothing used up.
Two quick checks. Your phone must support eSIM, and it must be carrier unlocked. Most iPhones from the XS onward and recent Samsung, Google Pixel, and other newer models work fine.