Skeletal formula is a way or representing the structure of an organic molecule. I doesn't have any symbols (e.g C for carbon, or H for hydrogen). Each straight line represents a carbon- carbon single bond, so it represents a carbon atom at each corner of the zigzag as well as the two ends of the zig zag. Each carbon atom is assumed to have 4 bonds in total which is why hydrogen isn't shown in the skeletal formula. In an organic compound each carbon atom may bond to another carbon atom (or two) so hydrogen atoms bond to the carbon atoms to make sure that each carbon atom has four bonds. Essentially, the zig zag is the carbon backbone of an organic molecule and the hydrogen atoms fill in the spaces.
The double line represents a carbon carbon double bond.
The hexagon is a ring of carbons. It's the same principle as before except every carbon atom is bonded to at least two other carbon atoms. Hydrogen atoms bond to the carbon atoms to make sure that each carbon atom is bonded to four atoms (like before).
The way I did it was at every end and corner of the zig zag, I would put a C, for carbon. I would then fill in the remained carbon-hydrogen bonds to make it up to four bonds for each carbon atom.