Why is it unreasonable? It’s just a process, a fixed set of steps you follow, depending on the specific library you used to run it. The details depend on what type of ciphertext you have, as well as how secure your process must be against attacks. A toy project has very different requirements from software that is entrusted to handle monetary transfers, for example.
A good library will include examples of how to use it. For example, the cryptography package includes a RSA decryption example, which uses an existing private_key variable to decrypt ciphertext, given (in addition to the ciphertext) a padding configuration. The latter is necessary because there are multiple ways you can pad out encrypted data to fixed-length blocks.