How would I color fast at t-shirt?

Soak the finished t-shirt in 2gallons water & 1/2 cup salt. Make sure all the salt is dissolved in the water before immersing the garment. Let soak for 1 hour or so then rinse in cool water & let hang dry. Always wash the garment in cold water as well.

I've washed items like this with vinegar to help set colors.

If you really want it color fast warm water with salt & vinegar Soak for a couple of hours, rinse in cold water & always wash in cold water

This video tutorial shows one method you could try: https://www.youtube.com/watch?v=ZTK9s3vVqU0

This may be helpful https://www.popsugar.com/smart-living/How-Set-Fabric-Dye-Clothes-35015330