Do you know about HLSL or any other programming language or would you start from basically zero?
If you know about HLSL already, I think it should be no problem for you to get familiar with the ReShade language extensions while porting those complex shader.
However, if you know "only" about programming or other programming languages, I would suggest you start with a more simple shader to know how HSLS + ReShade works before you try porting a more complex implementation.
If you start from 0, I would suggest to read about HSLS concepts from the very beginning where also programming principles are explained. (I am sure you can find plenty using Google)
Starting at that point with a DoF shader might be a bit overwhelming.
Anyway, that is only my opinion
What ever it is, you can get started one way or the other