Pen & paper is a great tool.

Thing about naming your methods for what they do. If the names end up like "CreateAPlayerAndGiveHimAGunAndLoadTheLevel()" then it should be broken down into separate methods to do each part - think of each method as doing one job.

Re-using code in libraries is a good practice to get into - saves you time and also you have to make the shared code flexible or you can't share it.

I don't generally use design patterns deliberately, but having a well-structured design on paper first is my top tip. Then implementing it in an OO fashion where each entity/thing is a class, and operations on a thing are methods of its class.