This is a very simple problem if we recall that we can find the strike and dip of any plane given three points on the plane. We simply need to read the elevations of points from the topographic map. Actually, the problem is often even simpler than that, because we can often select the points we use, and if two of them have the same elevation we can draw a structure contour through the points with no other constructions necessary at all. However, it's a good idea to cross-check the results using other data since geologic structures are rarely smooth planes.
1. Find the strike and dip of the sandstone layer from the map at left. Outcrops are in brown.
2. For pairs of points at the same elevation we can draw structure contours directly. The more points on a line, the better.
3. In other areas we can use the three-point method to find structure contours.
4. Find dip by drawing a cross-section or by trigonometry.
Real geologic structures are rarely planar over very long distances. Don't use points very far apart. Construct structure contours in small areas and extend outward as long as the contours are consistent.
Since geologic structures are rarely perfect planes, a certain amount of scatter in your data is inevitable. If a contour is approximately linear but has slight kinks, or misses a few points slightly, you can probably consider it a straight line.
Be extremely wary of extrapolating structure contours!
In all likelihood, data over a large area will not be planar. You can determine local strike and dip, but over a large area you will have to treat the structure as non-planar.