I have Android Marshmallow on my smartphone and would like to install LineageOS onto it.

I have bought a new microSD card for my phone and this can be put into a USB adapter to connect to my PC.
I haven't rooted a phone before so I will need some real basic help.

Firstly, how do I backup my current rom onto my SD card?

Secondly, how can I be certain the LineageOS rom I download from a 3rd party site is legit?
Is there a sha256sum (checksum) I can do?

Thank You!!!

I didn't find the right solution from the Internet.
References
https://www.linuxquestions.org/questions/linux-mobile-81/how-to-root-your-android-phone-4175607727/
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