The first step is rewriting your DP recurrence into line form. separate terms that depend on j and terms that depend on i. Start with the recurrence and expand all cost expressions. Group terms by what they depend on.
If see something like: a[j] × b[i] + c[j], that's line form with slope a[j] and intercept c[j]. Sometimes you need algebraic manipulation: factor out common terms, complete the square, or rearrange inequalities.
The goal is always the same: isolate j-dependent terms (slope and intercept) and i-dependent terms (the x-coordinate where you query). Once have this form, apply CHT. Each previous state becomes a line. Each new state queries the hull at a specific x-coordinate.