Making a parser for a script

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BNF

I can't understand how to parse an input string using a written grammar using "BNF".

What I have tried:

Hello, I'm puppysss and built a simple script language with a lexer recently.
I was about to think about making a parser for it but my try made my head blown up. let me tell you an entire story of my idea for my own script...

1. Analyze a code with lexer for dividing words.
ex) "var a=1,b=1,c c=a+b print c" => "var\a\=\1\b\=\1\c\c\=\a\+\b\print\c\"

2. Build a grammar for a simple script using BNF or something ...

but I was hit by the wall of number two so googled many articles about "BNF"
thinking "it's pretty cool to success two" and
finally I understood "BNF". that was a method to describe a grammar.

however I was hit again by the wall of number three
that is how to parse an input string with a written grammar using "BNF"

Is there anyone who could explain that to me?