Having installed the recovery twrp and having adb the installation goes easy.https://wiki.lineageos.org/devices/i9300/install

Download a LineageOS zip file as Lineage 16.0 that comes with Android 9 (alternatively as the lineage guide shows you might want to try to build one yourself) to your computer

Download the google apps https://wiki.lineageos.org/gapps.html to your computer

Download the Lineage extras https://download.lineageos.org/extras to your computer

Put the android device in recovery mode using the special button press power up sequence or ./adb reboot recovery to get twrp

Consider to backup the existing system ROM

Do Wipe and Format Data

Go to Advanced Wipe and select Cache and System and then wipe

Do advanced ADB Sideload on the device

On the computer do adb sideload <filename>.zip to move the file to the device and repeat this for all files

Use twrp to reboot the system