Start with the largest and work down
Pillow arrangements fall apart when everything is the same size. Pick the biggest pillow first, then add smaller ones in front of it. Two pillows the same size sitting side by side is the most common mistake and the easiest to fix.
On a sofa
A two-seat sofa takes two or three pillows. A three-seat sofa takes three or five. Put the larger squares in the corners and one smaller pillow or a lumbar in the middle, and leave the seat you actually sit in clear.
On a bed
Work in rows back to front: sleeping pillows against the headboard, then one row of decorative squares, then one lumbar or a smaller square at the front. A double bed carries one decorative row comfortably. A king can carry two.
Sizes that combine well
Squares in the 18 to 22 inch range are the workhorses. A lumbar around 12 by 20 inches breaks up a row of squares and is the piece most arrangements are missing. Anything under 16 inches tends to disappear against a full-size sofa back.
Cover and insert
A cover holds its shape when the insert is slightly larger than the cover — an 18 inch insert in an 18 inch cover reads as full rather than flat. Check whether an insert is included, because covers are frequently sold on their own.
What to skip
Any pillow you would move to the floor to sit down. If it comes off every evening it is not decorating the room, it is decorating the mornings.