Use sine when the angle is opposite over hypotenuse: sin(θ) = opposite / hypotenuse
Use cosine when the angle is adjacent over hypotenuse: cos(θ) = adjacent / hypotenuse
Use sine if the side you need is opposite the given angle
Use cosine if the side you need is adjacent to the given angle
Use sine for y-coordinate on the unit circle
Use cosine for x-coordinate on the unit circle
Use sine in SOH
Use cosine in CAH
Identify the reference angle first
Match the known side and the unknown side to the ratio
Use sine or cosine based on which two sides are involved with the angle