C++ - using glfwGetTime() for a fixed time step

After doing some research and debugging in my C++ project with glfwGetTime(), I'm having trouble making a game loop for my project. As far as time goes I really only worked with nanoseconds in Java and on the GLFW website it states that the function returns the time in seconds. How would I make a fixed time step loop with glfwGetTime()?

What I have now -

while(!glfwWindowShouldClose(window))
    {
            double now = glfwGetTime();
            double delta = now - lastTime;
            lastTime = now;

            accumulator += delta;

            while(accumulator >= OPTIMAL_TIME) // OPTIMAL_TIME = 1 / 60
            {
                     //tick

                    accumulator -= OPTIMAL_TIME;
            }

}