"just needed to do" means:
- give a lot of free time to think about concept
- think about rough design
- know how API works and know how it should interract with user
- spend a lot time for coding, forget about your main work and family if you have one
- polish things, but before that think about every fckin function in your code while developing so you can fix things faster
- finally polish interface and usability based on your style of workflow (so you should use it everyday)
- then promote it enough to get some cash for the efforts
I doubt someone so brave to do such thing, you maybe? Since you say "it is already exists, just need some small ui fix".