How to make your own washing powder | eHow- youtube how to make washing powder lemon washing soap ,Fill a large bowl with 2/3 to 3/4 of a gallon of water. Hold a cheese grater or rasp over the bowl and begin grating the soap into the water, stirring to prevent the soap from sticking together.How to make your own washing powder | eHowFill a large bowl with 2/3 to 3/4 of a gallon of water. Hold a cheese grater or rasp over the bowl and begin grating the soap into the water, stirring to prevent the soap from sticking together.