Iterative/Incremental Development  
The generally accepted best practice is to develop your programs using an iterative, incremental process..

Start with working program  
If there is a program which is similar to the desired program, many professional programmers will start with the existing program and make alterations..

Iterative/Incremental Development  
For large programs this is typically divided into two parts: unit testing tests individual modules and integration testing sees if the total program works when all the modules are run together..