I build it 1st & plan it later. What I mean by that is; I plan it out in my brain & then try the track before I nail it down. There's 1 advantage I have over most engineers of layouts, I always have plenty of room to make mistakes.
When I do metal art work, I never draw a picture of what I'm going to make, I just make it & 99% of the time it turns out just how I pictured it.
I tried many times over the years to draw out a plan, but when I started building it nothing ever looked like the drawings, so, I stopped that procedure after my 2nd layout.
This layout I'm building now is the ultimate for me. No grades, lots of mainlines, using all of my buildings off my other layout & still will have plenty of room to add more. Long & tall scratchbuilt bridges.
So, some people need to plan things out way ahead of time, not me.
Like a builder here in my town says, "cut it twice & measure it once".
If I have to take time planning something, that's time taken away from the actual construction. That's my 2 cents WOrth.
larry