These are the steps I followed to root my old Samsung Galaxy J6 device. The procedure might vary slightly for other devices

Requirements

• Stock ROM of the current device OS
• Android Development Bridge (ADB) installed in computer
• Heimdall in computer
• Magisk app installed in device

Process

1. Extract boot.img from the current ROM
   1. Download the ROM online from SamFW or GalaxyFirmware. It’ll be a .tar.md5 file
   2. extract the AP the file using tar -xvf.
   3. From the extracted AP get the boot.img.lz4 by the same method
   4. lz4 -d boot.img.lz4 to get boot.img Instead of getting the whole ROM, one can just download the AP file to start with.
2. Connect the android device to the computer via ADB
3. Send the boot.img to the Android device. Can use adb push boot.img /sdcard
4. Select Install in the Magisk app and select boot.img and get a patched boot image
5. get the patched boot image back to the computer adb pull /sdcard/Download/Magisk-...img
6. Install heimdall/Odin in computer
7. Put the device to download mode by adb reboot bootloader
8. When the device boots into bootloader mode, verify heimdall detects it by heimdall detect
9. When the device boots into bootloader, flash the boot image using heimdall heimdall flash --BOOT Magisk-...img
10. The android device will auto reboot. If Samsung Knox triggers and ask to factory reset, proceed with it and reinstall Magisk app.
11. Follow the instructions in the Magisk app.

Later I also managed to install Magisk in a 2017 J5 Pro following the Magisk wiki