The baby squash wither and fall off for one of two reasons: hot weather and drought stress, or because they were not pollinated. You can't do much about the heat, but you can water if the soil is dry a couple of inches below the soil line. In the case of poor pollination, there may not be enough bees or other pollinating insects available. It is possible to pollinate by hand. Find a male flower (the ones with long skinny stems) and dip a paint brush into the pollen. Then gently brush that pollen onto the stigma of the female flowers (the ones with fat little stems).