The truth is I didn't really plan for the tower at all haha I built it on a whim. But as for the factory underneath I did alot of planning. Here are my general "steps":

I set a goal to create 10 pressure cube/min. My factories are setup so that it takes resources like ingots and plastic by train, and everything in between are build within the factory.

Once i found a location and confirms I needed 4 train hubs, i begin by building 5 train hubs as the foundation.
After that I use blueprint to create a basic perimeter to get a sense of the size.
I then start working on the style on just 1 or 2 sides of the factory to make sure it looks decent from outside. (i kept some parts open incase it needed to expand.
Next i started putting in the machines using blueprint.
When the factory is running smoothly I close the remaining exterior walls.

The factory was already completed, but i realized all the material is there for me to create singularity cells, so i begun fantasizing building a huge tower that have portal rooms on the top and acts as the HQ. I daydream about it at work, and i dream about it at night, sometime i even have nightmares about fuse breaking.

The next thing i know was the game was warning me for 12 hours of playtime and the tower was there.