The ljust method doesn't do what you expect. The documentation says:

Return the string left justified in a string of length width. Padding is done using the specified fillchar (default is an ASCII space). The original string is returned if width is less than or equal to len(s).

Your list comprehension is correct.

One solution might be to use format. A good tutorial here.

Example code with format:

example_string = '''hello there how are you doing!
i am doig well thank you
lets get to work!!! '''


def modify_example_string(example_string, ch, n):
    new_string_list = ["{} {}".format(ch * n, element)
                       for element in example_string.split('\n')]
    example_string = '\n'.join(new_string_list)
    return example_string

print(modify_example_string(example_string, "*", 2))
# ** hello there how are you doing!
# ** i am doig well thank you
# ** lets get to work!!!