I think it is easiest to put some type of weighted container (like a small pyrex dish or plastic dish with a bit of gravel) in the tank then use a tube or some tweezers to put the food in the container. That way you can remove the food easily and know exactly how much of it they are eating.
I've been feeding my shrimp every couple days and I've been trying out different foods. So far I've tried Hikari Shrimp Cuisine, Crab Cuisine (they really like this), and Sinking Wafers, API Algae Wafers, Marine Select Green Marine Algae with Garlic Extract, as well as blanched spinach and zucchini (they didn't get much of the zucchini because the snails I didn't know I had all came out of hiding as soon as I put it in and ate it). Their favorite is the crab food. They honestly didn't seem to care for the shrimp food lol. Only one of them ate it! But they may have just had low appetites since they are still getting used to their new environment.