How can i do custom author list?

I assume that you would be providing the author IDs in the widget options. And that the authors would be displayed in the order they were listed.

Assuming the input would be -> 3,10,12

You can have the following code to display the authors with that user ID in that order:

$user_ids = "3,10,12"; //this is assuming you already have the value stored in a variable

//convert the comma separated string into an array
$user_ids_array = explode(",", $user_ids);

//display the list of users
echo '<ul>';
foreach( $user_ids_array as $id ):
    $user = get_userdata( $id );
    echo '<li>'.$user->ID.' '.$user->display_name.'</li>';
endforeach;
echo '</ul>';