How to Construct a Lattice from Program Statements
There is no such thing as a "DataFlow Graph lattice".  Those are two separate things.  A data flow graph is a graph that represents how data flows in a program, which can be helpful in data flow analysis.  A lattice is a mathematical object that can be helpful for data flow analysis.

Data flow analysis is a broad subject.  There are many techniques for data flow analysis (many use lattices, but it's not a hard requirement).  They don't all use lattices in the same way.  So, there's no one way to do the conversion you are referring to.

To learn more about the subject, I suggest reading a good textbook on program analysis.