For DTF prints on shirts, the goal is to have as much "shirt color" as you can in the design, so it is not solid ink on the shirt.
Before starting the following steps, be sure:
You have the desired colors (check CMYK values)
Black should be: C50, M50, Y50, K100
Fonts > Create outlines
Figure out how wide (what size) you want your design (how many inches?)
Get your final design on its own layer (make a copy, so you keep the original!)
Make sure “shirt color” is the same color everywhere (change to new (unique) color to be safe)
Select all > Object > Expand Appearance > Expand (a second time)
Select all > Pathfinder > Divide > Right click > Ungroup > Deselect
Select one item of the color you want to remove, then: Select > same > fill color > delete (on keyboard)
When you change the background color (shirt color), parts of the design should change also!
Save your Illustrator file with background turned off, and only your final design visible!
Ctrl-A (select all) to identify all objects!