My track layout ideas typically start by scribbling on a notepad. All you really need to do is imagine a layout that will be cool to watch in action as the trains go round in circles and then draw it in rough. Do not be pressured at this point however because this will almost never be the finished idea. What happens next is that you set up the idea and as you do this you will get some "light bulb" moments and figure out some customization that makes the whole thing look better.
A lot of the time when building track layouts for your model trains to run on you will do the most changes when placing the landscape around the track. You will end up disconnecting and then connecting parts all over the place. This process is really fun and when the whole thing actually starts to take shape. When you become more experienced then your track layout ideas in your head will actually start with the buildings, trees and other landscape features already in your head.
Building tracks around the landscape
Some railroading enthusiasts like to first lay out the landscape features and construct the train tracks around them which is just another way of doing things. I prefer to do a mixture of all of what is discussed above as it means I am never limited to doing things any one way and have certain flexibility to my process which helps me be even more creative.