Symmetric Encryption Analysis & Decryption Methodology

I have a decrypted string and its corresponding encrypted string, and I need assistance in analyzing the encryption method and describing it to me in a way that I can use to decrypt future strings.

Key Details:
- I suspect the encryption method used is symmetric encryption.
- I do not have access to any private or public keys used in this encryption.
- The format of the encrypted string is Base64.

Ideal Candidates:
- Expertise in cryptography, specifically symmetric encryption.
- Ability to explain complex concepts in simple terms.
- Experience with Base64 encoded data.