Do I need to prepare plain text before encryption?
If you are using a modern, secure cipher, there is no reason whatsoever to perform such manipulations of the plaintext.

I'm not sure how to elaborate much more on the topic than that. The entire purpose of a cipher is to perform the exact types of confusion and diffusion you describe. Only, the approach that it will use has been designed, peer-reviewed, and cryptanalyzed by professional cryptographers. Your hypothetical algorithm has not.

At best case, it will do nothing to improve the security of your ciphertexts. At worst case, you can introduce bugs that unintentionally cause plaintexts to be unrecoverable from the ciphertext, or ones that accidentally reveal information about the plaintext (for instance, via a leak of timing information).