Hold down your finger anywhere on your screen (or on an app and select "Edit Home Screen") until the apps wiggle.
Tap the + icon in the top left corner.
Select the Smart Stack widget, pick a size (small square, medium horizontal rectangle, and large square), and tap Add Widget to add it to your home screen.
Move your widget to the location you want on the home screen and then select Done in the top right corner to set it in place.
You an also create your own Smart Stack widget by dragging up to 10 other Apple or third-party widgets of the same size on top of each other.
To remove apps from Smart Stack press down on home screen until apps wiggle and tap Smart Stack, swipe from right on app you want to remove and select Delete. Or hold down finger on Smart Stack and select Edit Stack.