An iron key is to be plated with nickel…

Step 1. The key. Nickel ions must be reduced to nickel on the key, and reduction happens at the cathode. So the key is the cathode.

Step 2. The half-reactions. At the key: $\text{Ni}^{2+}(aq) + 2e^- \rightarrow \text{Ni}(s)$. At the nickel bar, which is the anode: $\text{Ni}(s) \rightarrow \text{Ni}^{2+}(aq) + 2e^-$.

Step 3. The terminal. The cathode is joined to the negative terminal of the battery. So the key goes to the negative terminal and the nickel bar to the positive one.

Step 4. The solution. One nickel ion is made at the anode for each one used at the cathode. The concentration does not change.