Either in a bathtub or in 2x in a large basin, soak for the first time with 15-20 ml of laundry detergent in temperate water (about 20-30 degrees) by stirring and rubbing the inside.
Rinse by filling the basin, empty it, and start again with hot water (about 40 degrees not to burn) with 50-60 ml of laundry detergent and rub well by folding in half to rub on itself the cloth diapers or inserts and soak a little, then rinse everything well (pass under the shower spray first, then soak in clear water to finish well) and dry.
(For the laundry detergent dosage, see according to the brand, and the amount of water. The data put here is an example).
If it's something you plan to do often, a washboard for good rubbing is a good buy or with a diver to brew in a bucket as follows: