Creating 3D Ballparks

3D Parks are best created using SketchUp, a free 3D editor. Currently it is a web-based tool, although a legacy desktop version ("SketchUp Make 2017") can be downloaded from the Internet Archive (please note that the following links may break at any time):

While the process of creating a 3D ballpark from scratch is rather involved, forum user silvam14 created a series of YouTube Videos detailing the process:

Part 1

Importing the diagram
Scaling the diagram
Outlining the playing area
Creating the fences
Using the "follow-me" tool to create the stands
Cleaning up the dugout areas and other parts of the park

Part 2

Create faces for CF scoreboard
Fix Outfield Fence Face for future texturing
Open up center field back wall below roof
Place supports between roof and upper deck

Part 3

Painting Textures
Importing and Re-sizing textures

Part 4

Exporting the model from sketchup to an .obj
Importing and testing the stadium in OOTP19 (fixing anything wrong)
Configuring the 3D crowd and testing in OOTP19
Creating the 2D diagram
Creating the 2D Day and Night Photos
Setting The 3D Grid and wall markers
Setting 2D Ball locations
Creating park factors
Exporting park .prk file

For more information, see silvam14's thread on the OOTP Forums.