To tuck a top into your bottoms when styling a look, tap on the Tuck button!

The 'Tuck' button appears when players use pants, skirts, and shorts with a top.

The 'Tuck' button can be found at the lower right side of your Model screen, beside the Seasonal and Unworn bonus bars.
  
Simply tap on the 'Tuck' button and you will see your outfit tucked in. Tap again to untuck your top.

Using the 'Tuck' button can totally transform your outfit. Give it a try!