For a parametric curve (x=x(t)), (y=y(t)), find (frac{dy}{dx}=frac{dy/dt}{dx/dt})
A horizontal tangent line occurs when (frac{dy}{dx}=0)
Set (frac{dy}{dt}=0)
Make sure (frac{dx}{dt}neq 0) at the same (t)-value
Solve for the parameter (t)
Substitute that (t) into (x(t)) and (y(t)) to get the point
Write the horizontal tangent line as (y=text{constant}), using the (y)-value of the point
Check that the point is on the curve and that the tangent is not vertical at that (t)