With Lasso Fill, you can easily fill in the area you have boxed in with the lasso. In this example, we will give the character a "cartoon-like" shadow.
Open ①Layer window. Tap ②Add Layer to add a layer. Set ③Blend Mode list to "Multiply".
Open the ①Tool Select window and tap ②Special.
Open the ①Special Pen window and select②Lasso Fill. Adjust ③Opacity as you see fit.
Box in the area you want to add the shadow and fill it in.
Finally, open ①Layer window. Select ②shadow layer and click ③Clipping.
And we are finished!