How to explain the steps in the implementation of hashing functions like SHA1
I recommend you spend some time with some crypto textbooks, in particular the Handbook of Applied Cryptography and Antoine Joux's Algorithmic Cryptanalysis.  They explain some of the details of hash functions.  You might also try reading some of the questions on Crypto.SE; there's a lot written there.

In addition, do some self-study about unbalanced Feistel networks.  They are part of the core of SHA1.  You can find information about them on Wikipedia and in the cryptographic literature.

You might also like to read the submissions to the SHA3 competition.  Every one came with a detailed technical document that explains the design criteria and security analysis of the hash function.