When I tuck in a sweater, I start front and center. I make sure the sweater is lined up correctly with the center of my skirt, jeans, or pants.

For this look that got all the questions, my sweater was only tucked from center to where the pockets start. The rest of the sweater was untucked and just bloused under.

For this look, my sweater had slits and I only tucked in the front part again. By doing this, the front ended up being tucked in beyond the slits, which helped the back part blouse under on its own.

Most of us curve in at the small of our back and curve out on the opposite side. By having the bit of bulk in the back of your look, it balances everything nicely.

I tucked a sweater into a skirt that was fitting a bit snug that morning. You can see from this side view that I kept the sweater completely smooth and let the gathering happen at the small of the back. I bloused it out a bit to make space for the gathering. 

You can see how I pulled the side volume to the front and it almost created an architectural detail. It created more of a waist shape.

I only tucked in a couple of inches of this thick sweater. Any more, and the drape would be lost and with it my shape and my comfort.

This sweater was fully tucked because the spongey weave of this sweater, while thicker than linen, stretched easily. Just by gathering the bulk at the small of the back, it still achieved a high-low effect.