Adding new fields using hook_field_schema_alter() I have an entity whose field schema is already defined and now I am trying to add couple of fields to it.

But I am not sure how to code it up using schema_alter.Below is an example on how I would code the schema for the field using hook_field_schema().

function hook_field_schema($field){
  $columns = array(); 
  if($field['type'] == 'some_field_name'){
  $columns = array(
              'col1'=&gt;array(
                     'type =&gt;'text'
                     'size' =&gt; 'big'
                     'not null' =&gt; false),
              'col2'=&gt;array(
                     'type =&gt;'text'
                     'size' =&gt; 'big'
                     'not null' =&gt; false),
              );
  } 
 //Similar for other fields  
}


Confusion is on how to decompose this schema according to schema_alter.

I am following this article to implement hook_field_schema_alter()

Edit: I am using an update function to create the field (field_create_instance), since this is an already an existing module and I do not want to disable it and enabled it again.
You would need to call the field_create_instance() function passing a new array.

 $instance = array(
    'field_name' =&gt; 'field_my_custom', // ** This already exists, or check with field_info_field()
    'entity_type' =&gt; 'node',
    'label' =&gt; 'a label', 
    'bundle' =&gt; $content_type, 
    'widget' =&gt; array(), // if you're not sure about your widget settings, take a glimpse at an existing field instance using debug(field_info_instance(...));
);
field_create_instance($instance);


More links to read and better understand the above functions:

https://api.drupal.org/api/drupal/modules%21field%21field.info.inc/function/field_info_field/7
https://api.drupal.org/api/drupal/modules%21field%21field.info.inc/function/field_info_instance/7
https://api.drupal.org/api/drupal/modules%21field%21field.crud.inc/function/field_create_instance/7

Also, take a look at the Examples for Developers module, node example and field example.