Steps: (1) Identify the symmetry (cylindrical for wires, rectangular for solenoids). (2) Choose an Amperian loop along which $B$ is either constant or zero. (3) Evaluate $\oint \vec{B} \cdot d\vec{l}$: for constant $B$ parallel to $dl$, this is $Bl$. For $B \perp dl$: contribution is zero. (4) Find $I_{\text{enclosed}}$ by the loop. (5) Apply $Bl = \mu_0 I_{\text{enc}}$ and solve for $B$. Common Amperian loops: circle for wires, rectangle for solenoids.