How to create manually create my own ᴏᴛᴀ update file to be used for adb sideload?
You can't create or modify OTA zip without the proper OEM signing key. However, there exist holes in older devices like Samsung Galaxy Y which allow flashing unofficial zip with test keys, but these kind of hacks are outdated (source)  

if you want to create OTA zip you need update-binary for your device and write updater-script in EDIFY language  

you can also replace update-binary with shell script which makes things easier and device independent  

learn how to sign your zip (scroll down to Zip Signing)