Step 1: [Ni(CN)₄]²⁻ has cyanide (CN⁻), a strong-field ligand, which pairs up nickel's d-electrons and gives a flat, square planar shape.
Step 2: [NiCl₄]²⁻ has chloride, a weak-field ligand, which doesn't force pairing, so it keeps a tetrahedral shape.
Step 3: So [Ni(CN)₄]²⁻ is square planar, and [NiCl₄]²⁻ is tetrahedral.