Programming pattern/strategy to handle big programming project in Unity3d

Almost three months ago! We started a big Tough project and still it is continue almost done, some bugs, some logic issues and some redundancies in code. Its a Unity3d Project. Every models/animation/environment are all completed nicely then, it came to the programmers(us).

Now, after writing too many codes, some ill logics, bugs I have learn so many things and I don't want to share. Its is scatters in my mind strongly disagree with lead/teammates but anyhow it is continue. But the next big thing I want to ask you that how do we handle a big project(programming side)? What are the best right suitable pattern to program in Unity3d using C#. How experience programmer do this work? how do they start to code? What I did wrong or notice wrong is the list given below:

 1. Did not do paper work enoguh i.e., algorithm writing

 2. change code abruptly

 3. write code abruptly

 4. without plannig code

 5. things make dependent

Guide me the right thing so that next time it should keep in mind and we don't made any mess.