Tools you need:
» First open "Color Magic"
» Choose the "The Sims 4" option then enter your creator identity and click "Next"
» Now you must choose where will you put your image, I chose a child t-shirt
» It's time to choose a unique name for your t-shirt, click "Add", because we will need this image later, and click in "Save as", this will save your package.
» Now open S4PE and opens the package file you just save
» Now you have to export the textures, right click on "UNKN" and go to Export to DDS and save the textures
» It's time to open the textures we exported in Photoshop. To change the green, you have to go to channels, and switch the green channel, what is gray to black
» Now just add the image you want, then flatten the image (layered - flatten image), we can only keep one layer
» Save your texture. Press Save as and choose .dds format
» We still have to change the image that we export first, we copy the same image that we added to the t-shirt and pasted on top of this image, flatten the image (layered - flatten image) then save.
» Let's return to S4PE, then import the file we saved (t-shirt with the image)
» Replace the image, then save in Documents/ Electronic /The Sims 4/ Mods
» It's time to open the Sims 4 Game and see the result
To add another image, you have to repeat the whole process from the beginning
I hope you find this tutorial useful, if you have questions put in the comments, I try to answer as best as possible and as fast as possible