To prove Snell's Law using Fermat's principle, start by expressing the sines of the relevant angles in terms of your chosen variables. Apply Fermat's principle to the time expression you've created, which involves calculating the path taken by light. It is essential to express the velocities in terms of the indices of refraction for the two media involved. The proof will involve significant algebraic manipulation to match the sine expressions. Following these steps will lead to a successful demonstration of Snell's Law.