I feed the clams by filling a syringe (no needle) with the puree solution. You can slowly allow the food to fall from the syringe and onto the clams.
It takes a bit of trying out but eventually you'll get it. The clams should start feeding almost immediately. It helps to keep the feeding solution close to the temperature of the clam's water.
You can also place them in a bowl with their tank water and add food. Keep it stirred every so often. Leave them in there for about 30 or 40 minutes. They should feed well that way too.
Be careful of chlorine in tap water, my clams came into contact with too much and it killed them! A bit of 2 A.M thoughtlessness on my part destroyed my healthy babies. :(
I measured my clams monthly. They showed noticeable growth each time.