Choosing the right Gaussian surface is the key skill. Rules: (1) Spherical symmetry (point charge, sphere) — use a concentric spherical surface; E is constant on the sphere and perpendicular to it; (2) Cylindrical symmetry (infinite line, cylinder) — use a coaxial cylindrical surface; E is constant on the curved surface and perpendicular to end caps; (3) Planar symmetry (infinite sheet) — use a pillbox (short cylinder) straddling the sheet; E is perpendicular to the flat faces. On the chosen surface, E . dA simplifies: if E is parallel to dA, it becomes E*dA; if perpendicular, it's zero. The surface must pass through the point where you want E.