(1) Draw a horizontal line. This represents the boundary between the two media with different indices of refraction. Draw a dotted vertical line perpendicular to the horizontal line that bisects it. This represents the line normal to the surface of the boundary between the media.

(2) Sketch the incoming ray. Remember that the angle in Snell's Law is relative to the normal, so make sure you read the problem carefully to determine if you were given that angle or an angle relative to some other line.

(3) Use Snell's Law $$n_1 sin \left( \theta_1 \right) = n_2 sin \left( \theta_2 \right) $$ and solve for the unknown varaible (you will be given some combination of 3 of the 4 variables $n_1$, $n_2$, $\theta_1$, or $\theta_2$, and asked to solve for the fourth).

More challenging Snell's Law problems will involve solving for the index of refraction or angle of refraction for some third medium. If you are given three media instead of 2, just split the problem into two stages and do the 3 steps above twice: first for $n_1$ and $n_2$, and then for $n_2$ and $n_3$ (similar to using $v_f$ from one stage of a problem as $v_i$ in the next stage).

Here is an excellent experiment to verify the laws of refraction and determine the refractive index of glass using Snell's law.