how to make a Turing machine of a cipher?
Honestly, the answer to "How do I model X using a Turing Machine is:"


Implement the algorithm using a programming langauge, probably a simple one like lambda-calculus
Translate from that language into a Turing Machine


We could tell you how to write a machine that does a Caesar cipher, but it would just be us doing the above steps for you.

Turing Machines are very un-intuitive to program in, and they're usually not used to model algorithms directly, more as a theoretical baseline of what an algorithm is. But while there are lots of proofs that model Turing Machines in general, usually you don't deal with specific machines.

If you really need to model things directly with TMs, try using a variant of them, such as RAM machines (which would probably be much easier for modelling a Caesar Cipher), and/or multi-tape TMs, then use the known translations to ordinary machines.