Answer
The lewis structure is drawn the way the formula is written. The atoms next to each other are connected to each other. $CH_2$ is connected to CH with a double bond and this CH is connected to another CH. The last CH is connected to the Oxygen with a double bond.
Work Step by Step
The condensed formula is $CH_2CHCHO$. The first C is connected to two hydrogens and other C, and this makes 3 bonds. But, carbon forms 4 bonds so the bond between first C and second C is a double bond. The third C is connected to an Oxygen (with a double bond) and a hydrogen. There are two lone pairs on the oxygen.
