A Lewis symbol is a symbol in which the electrons in the valence shell of an atom or simple ion are represented by dots placed around the letter symbol of the element. Each dot represents one electron.
Hydrogen
Oxygen
Chlorine
Chloride ion

A covalent bond is a chemical bond formed by the sharing of a pair of electrons between two atoms.

The Lewis structure of a covalent compound or polyatomic ion shows how the valence electrons are arranged among the atoms in the molecule to show the connectivity of the atoms.

Instead of using two dots to indicate the two electrons that comprise the covalent bond, a line is substituted for the two dots that represent the two electrons.

Below is shown the Lewis structure for water. Two hydrogens (H) are separately covalently bonded to the central oxygen (O) atom. The bonding electrons are indicated by the dashes between the oxygen (O) and each hydrogen (H) and the other two pairs of electrons that constitute oxygens octet, are called non-bonding electrons as they are not involved in a covalent bond.

There are several steps to keep in mind when drawing a Lewis structure.

1.  Determine the number of electrons to be used to connect the atoms. This is done by simply adding up the number of valence electrons of the atoms in the molecule.
    Consider carbon dioxide CO2
     
    carbon (C) has four valence electrons x 1 carbon = 4 e-
    oxygen (O) has six valence electrons x 2 oxygens = 12 e-
    total = 16 e-

    There are a total of exactly 16 e- to be placed in the Lewis structure.

    2.  Determine the connectivity.  The unique atom is usually central.  Avoid long chains.  Connect the central atom to the other atoms in the molecule with single bonds.

    Carbon is the central atom, the two oxygens are bound to it.

    3.  Complete the octet of all the atoms in the structure.

    4.  Remove electrons, from the central atom, in excess of the number calculated in step 1.

There are 20 electrons in this example, but only 16 needed and required.  Remove 2 pair from the carbon.

5.  If the valence shell of the central atom is complete, you have drawn an acceptable Lewis structure.  If not, the electron deficiency (below an octet) is made up by sharing lone pair electrons from the outer atoms to make double or triple bonds.  Continue this process of making multiple bonds between the outer atoms and the central atom until the valence shell of the central atom is complete.

Carbon is electron deficient - it only has four electrons around it. This is not an acceptable Lewis structure.
 
becomes

The central atom is still electron deficient, so share another pair.
 
becomes

The best Lewis structure that can be drawn for carbon dioxide is:

6.  Calculate formal charge (FC) on each atom.

FC = # of valence e- - (# of lone pair e- + 1/2 # of bonding e-)

The formal charge on each atom in CO2 is zero, so a different example will be used.

Consider the molecule H2CO2. There are two possible Lewis structures for this molecule. Each has the same
number of bonds. We can determine which is better by determining which has the least formal charge. It takes
energy to get a separation of charge in the molecule (as indicated by the formal charge) so the structure with the
least formal charge should be lower in energy and thereby be the better Lewis structure.
 
FC (H) = 1 e- - (0 e- + 1/2 (2) e- ) = 0
FC (C) = 4 e- - (0 e- + 1/2 (8) e- ) = 0
FC (Oa) = 6 e- - (6 e- + 1/2 (2) e- ) = -1
FC (Ob) = 6 e- - (2 e- + 1/2 (6) e- ) = +1
FC (H) = 1 e- - (0 e- + 1/2 (2) e- ) = 0
FC (C) = 4 e- - (0 e- + 1/2 (8) e- ) = 0
FC (Oa) = 6 e- - (4 e- + 1/2 (4) e- ) = 0
FC (Ob) = 6 e- - (4 e- + 1/2 (4) e- ) = 0

The two possible Lewis structures are shown below. They are connected by a double headed arrow and placed in brackets. The non-zero formal charge on any atoms in the molecule have been written near the atom.
 

The two structures differ only in the arrangement of the valence electrons in the molecule. No atoms have been
moved. These are called resonance structures. The better Lewis structure or resonance structure is that which has
the least amount of formal charge.

The bond order is equal to the number of bonds between two atoms.

The bond length is the distance between those two atoms. The greater the number of electrons between two atoms, the closer the atoms can be brought towards one another, and the shorter the bond.

The BO is an indication of the bond length, the greater the bond order, the shorter the bond.