not trusting just because you feel paranoid doesn't make any sense specially since all the good projects that also happen to be popular like bitcoin core and electrum are 100% open source so there is no room for not-trusting since you can see everything the tool does.

but if your purpose is to learn more about bitcoin and how things work under the hood then that's a different thing. if it is the learning you seek then tell us if you have any background so far, for example how much do you know about transactions, their serialization, scripts, OP codes, cryptography and public/private key pairs?
i can write a step by step guide starting from 2 private keys all the way down to the scripts they create and their corresponding addresses but you have to know the basics i mentioned.