How to write a palindrome sequence as-wish (create, theoretically) ? (self-answered)
I once developed a method, but it is so-basic and simple that I'm pretty sure it is already discovered. Here I "shared my knowledge, Q&amp;A-style"

Step-1

Write a small sequence randomly. 

    5'... A T G C C


Step 2

Write the sequence in opposite-direction, on the next line. It will end at just at the next-place of last place of previous sequence, i.e. 

         --------&gt;


   5'... A T G C C

                        C C G T A ...5' .

                    &lt;-------------


Step-3

Fill-in-the blanks following base-pair rule, and the sequence is now ready.

5'... A T G$\:$ C C$\:$ | G G C A T ...3'

3'... T A C G G | C C G T$\:$ A ...5' 

Addendum (courtesy: user@Another'HomoSapien')

The previous method was similar way we usually read a verbal palindrome (though unlike a verbal palindrome, any reflection-symmetry was Not present ). But  there is a rotational-symmetry (2-fold), (rotational symmetry means it contains parts which are superposable on rotation). 

So we could create a palindromic sequence using rotation also.



Or could rotate pre-filled up sequence (if exactly follow the user's instructions)



The brown dot indicates the axis of rotational-symmetry of the written-sequence, vertical to the plane of paper.