What is the most effective way to clean lampin? Especially if it's white?

Sunning is an effective way to whiten clothes. I can attest to that. Sometimes, whenever I prewash my baby's cloth diapers and some poop stains remain, I just line dry it under the sun and it disappears like magic thereafter. Generally, lampin is easier to clean. You may also soak them using Cycles Stain Soaker. It's a detergent made for babies.