Unless you're building a temporary table-top layout, like my Kitchen Table Layout, advance planning is critical to constructing a layout you'll enjoy. There are many good books on the subject, and probably plenty of online opinion pieces as well. What I'm going to do here isn't another of those. I'm just going to describe the process I went through, and the various decisions I made, and as best I can determine why I made them. Some of this was written within six months of the original design phase, but not all of it, and memory fades with time.
I'd started with U.S. prototype HO-scale freight many years ago, then decided I wanted to do something different. The first layout was my Kitchen Table Layout (KTL), which was a simple oval of double-track I gradually evolved until I ran out of table. That started me thinking about what I wanted to have, and how much of that I could fit in my very limited basement.
The Want List came first, and that led to the idea of a layout composed of scenes, which also worked well with my evolving desire to use a repeatable sectional design similar to a modular layout to permit the whole thing to be relocated for inevitable moves (I've read of far too many layouts that had to be destroyed when the owner moved, after decades of labor; I don't want that).
Along the way I've also come up with ideas that didn't fit, one of which became a design for a simple urban shelf layout you could fit along one wall of a typical room. Another became my Urban Tram Layout, which now sits atop a chest on one side of my Living Room.
I work on a Mac, and layout design programs are a bit limited there. I use Rail Modeler primarily. I've also looked at XTrackCAD but found the learning curve a bit steep; I may go back to it someday, as its ability to integrate with JMRI's automation capabilities is attractive.
But mostly, since I'm working with sectional track, I just stick things together and decide if I like the result, and if not I try something else. It's "prototyping" in the design sense, and a very useful technique, if you don't mind ending up with a large box of unused (and expensive track). I could sell it, but I'm sure I'll use it on the next project...