To draw the Lewis structure of covalent molecules/ions following steps are followed

(i) identify the central atom in the given species. Generally, central atom is that atom which is less in number, usually electronegativity of central atom is less. Hydrogen is never a central atom. 
(i) Atoms which are directly attached with central atoms are called surrounding atoms. (ii) Arrange the surrounding atoms around central atoms and make a single bond between 
each pair of central and surrounding atoms. If by doing so the octet of surrounding atoms is not complete, then make double or triple bond to ensure that the octet of all the 
surrounding atoms is complete, keeping in mind the covalency of central atoms. Usually covalency of any element is the number of unpaired electrons either in ground or in 
excited state. 
(iv) Covalency of any of the second period element can never exceed four i.e., it can't have 
more than eight electrons in its valency shell. However, covalency of 3rd and lower period elements can be less than, equal to or greater than four. 
(v) Represent the lone pair of electron particularly of central atom. 
(vi) If the given species is polyatomic ion then before making any bond between central atoms 
and surrounding atoms distribute the charge on surrounding atoms symmetrically as far as 
possible and then follow all the above mentioned steps