To start with I assume you realise you need one more dimension than those named in the function.

That is if you have F(x,y) the level curves or surfaces occur in the z dimension.

Since F(x, y) is not a function of z it does not vary in the z direction.

Another way of saying this is that z = a constant.

So level curves have the form F(x,y) = a constant.

You draw them by substituting the appropriate function and working out where (in this instance)

F(x,y) = xy = c

Each different c will yield a separate level curve (also called contours)