How to collect thoughts in a heap with a big refactoring?

Question

OOP

Hello.
At work, I got the task of refactoring the calculation kernel of one tool. The core was made in a hurry by another developer, so there are no unit tests, no algorithm extension points, no documentation, since there is a developer who did this and I consult with him on logic.
The kernel algorithm is quite large and it is difficult for me to consider it in its entirety. I broke it into several logical pieces and work with each one separately, while trying to apply TDD: when implementing some piece of logic, I ask myself: "How will I test this?"
What's the question? All these actions are difficult for me. I have no confidence in my actions. Are there any resources: articles or books that will help you be more confident in writing code requirements expressed in unit tests and organizing the design of complex calculation processes involving many if / else.
There are no seniors who I could consult on this issue (I tried), they are colleagues, either pure sql-schikov, or only students who arrived with little experience. I myself have been programming for only 1.5 years and have no experience in this.
If interested, I use the c# language.

Answer 1

Here I can advise so far only Fowler and his book about refactoring link .
There is another book on working with legacy code link , but its translation is so terrible that I would not advise it, but those who read it in the original say that it is very good!
I think that you can't get by with any advice and articles here, you have to read it.
Read on or you're doomed to failure!