In order to do successfully do these types of images "in camera" you ideally need an exposure of 15 - 30 seconds. Why? Because of the time need to hold back the sky but that will be clearer in just a tick.
Firstly, find yourself a gorgeous scene. Things like seascapes are good or reflections in water.
Meter the scene and determine the exposure for the foreground. As mentioned, you ideally need a minimum of 15 seconds. If you can't achieve this, try adding on one of those 8 to 10 stop ND filters to lengthen your exposure.
With your foreground exposure determined, you need to work out the sky.
From extensive experience of using grads, sunsets will usually need around 3 stops of light to hold back the sky.
Minus off 3 stops of light from the base exposure to get the time needed to burn in the sky.
If your base exposure is 30 seconds then the time needed for the sky is 4 seconds.
Now, go practice!!!
For times longer than 30 seconds, to save you time in the field counting things out, this is how it works out in full stops: