The Yamaha DX7 is known for being hard to program, it doesn't help that some common synthesis concepts work differently on this synth. Dexed is a good clone, that helps you get a good overview.
I suggest you check out my intro to FM synthesis first if the concept is completely new to you.

More resources:
3min intro to FM synthesis:    • Short Introduction to FM Synthesis in...  
Making IDM & Glitch with Dexed:    • Dexed / DX7 An IDM Glitch Dream Machi...  
Download Dexed: https://asb2m10.github.io/dexed/

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