Combine the very warm or hot water and the washing soda and mix until dissolved. Mix in a spray bottle for a liquid version, or in a bowl if using it as a paste.
Add the hydrogen peroxide.
Use as a pre-treat spray on stains. You can also add the entire mixture to some water in a small basin and let the stained clothing soak for 30 minutes before washing.
Notes
This is a liquid recipe and reminds me somewhat of Shout laundry stain remover spray.
You can also mix and use it like a paste for more stubborn stains.