There might be a few reasons that your apple chips turned out soft and mushy. First of all, the temperature on your oven might be slightly off. I'd recommend grabbing an oven thermometer to make sure it's accurate. Also, you might need to cut your apples a little more thinly -- I use this mandoline slicer to get those nice thin slices. Finally, make sure you're cooking your apples long enough -- the edges of the slices should be curled when they're ready.