Click the "+" icon and add two C (carbon) atoms. Each carbon atom will have four unpaired electrons.
Click the "+" icon to add four H (hydrogen) atoms.
Click on two unpaired electrons of one carbon atom and then on two unpaired electrons of the other carbon atom to form a double bond between the carbons.
Click on the remaining unpaired electron of each carbon atom, then click on the unpaired electron of a hydrogen atom to form single bonds. Repeat for all four hydrogen atoms until the structure is complete.