You can make 3% solution by mixing 11 parts water a 1 part 35% H2O2, total 12 parts (oz, cup, litre etc, whatever measure you want to use)
I would use 1 litre of 3% solution per warm bath or use 6 ounces to 1 cup of 35% solution in the bath. You could also add Epsom Salts and baking soda. Keep the bath water away from the baby's eyes.
I once experienced a baby tip over and sit in a gallon of 35% food grade H2O2 (Hydrogen-Peroxid) for about five minutes before she was discovered by her mother. Other than having a white bum for awhile and a bit of discomfort, there was no damage to be seen within an hour.
Hope this helps