Knowing what kind of layout you want is the place to start, and it sounds like you’ve already figured that part out (continuous running, large equipment). There are many different ways to figure out the rest, depending on what suits you.
If you have a fairly flat piece of land you can just start laying out stuff on the ground, using track, string, pvc pipes, etc. Then just play around with it until you come up with something you like.
Another way to go is the plot everything out on paper or using a computer program. Some folks like doing it that way, some don’t.
In my case, I had a difficult piece of land – long, skinny, and on a steep slope. So large equipment and wide curves were out. Which is fine, I like funky little narrow gauge railroads. I’m not real big on planning everything out in detail, I prefer to start with a rough idea and work out the details as I go. So that’s what I did.
I had to dig out a lot of the hillside and fill in other parts. I started at one end, and as I went along I would lay down pieces of sectional track to figure out what fit. Then I’d dig and fill a little more, and figure out the next stretch of track, and so on.
When I got to the far end, I did have a moment of despair when it seemed like I wouldn’t be able to fit everything together. But I just kept working with it until I found something that worked, and now that’s actually my favorite part of the layout.
So that’s how I did it. It won’t work for everyone but it worked for me.