Its not hard you can google it there are many guides if you search something like
"sign aab file using jarsigner"
here is a quick guide below what i've done using a Windows machine. You also need to make sure to have your keystore file to proceed
also you need to have java JDK installed
jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore [path to keystore] -signedjar [name for the output signed file].aab [path to unsigned aab] [keystore alias]
[path to keystore] should be the path where you saved your keystore .jks file
example: c:\aab\keystore.jks
[path to and name for the output signed file] should be the name you want it to create after its signed and which folder
example: c:\aab\appname-signed.aab
[path to unsigned aab] is the path to where your file is exactly
example: c:\aab\appname.aab
[keystore alias] name of the Alias you have given in your keystore
example: APPNAME
if you don't have a keystore you need to generate one very easy using Android Studio or APK Editor Studio.