Double bonds

Each oxygen contributes 2 electrons. The carbon contributes 4 electrons. The 8 shared electrons form 2 double bonds. We do not show the 1s electrons.
A carbon atom has 4 electrons in its valence (2) shell. A hydrogen atom has 1 electron in its valence (1) shell. The valence (2) shell of carbon holds 8 electrons when full. So each carbon had 4 bonds attached to it.