How to casually do the required Software Engineering and designing?

I am an inexperienced developer and face the following problem:

1. My company is a start up and has no fix Software engineering systems.
2. I am assigned tasks with not very clear and conflicting requirements.
3. I don't have to follow any designs or verify requirements officially.

Problem:

1. I code all day and finally get stuck where requirement conflicts and I have to start over again.
2. I can-not spend a lot of time doing proper SRS or SDD.

How should I:

1. List out Requirements for myself. (Not an official document)
2. How to verify and validate the requirements?
3. How to visualize them?
4. How to design them with minimum effort? (As its going to be with me only)

I don't want to waste my time coding something that's gonna collapse according to requirement conflict or something!

I don't want to compromise with quality but don't want to re-write everything on some change that I didn't expected.

I imagine making a diagram for my thought process that will show me conflict in the diagram itself, then finally correcting the diagram - I decide my design and structure my code in terms of interfaces or something.

And then finally start implementing my design.

I am able to sense the lack of systematic approach, but don't know how to proceed!

Update:

Please suggest me some tools that can ask me the questions and help me aggregate important details.

How can I have diagram that I talked about for requirement verification?