The trim “how to” process is not covered in detail simply because of the variations available in the plans, it would be next to impossible to cover the topic properly.
In a nutshell, you just get out the old tape measure and custom fit your pieces similarly to those shown in either the drawings or from what you see in the builder’s gallery. The style of trim can also vary greatly from store to store, so just shop around for what fits your style and budget.
This is the phase of construction where you can get creative.