Nobody sits down and writes much good work just by deciding to do so. What a writer needs to do is be ready like a song writer for something that can be bridged to and from. If you examine a lot of good films you will know that there is one or two ironies or spins the writer probably encountered in their own life and then the whole thing was about how to lead up to that and have it be dramatic or funny or w/e and then have it resolve to make people feel like they are satisfied with the entertainment value. That's why they say "you can't make this stuff up". Stuff that is not based on reality comes off as contrived and critical thinkers will see right through it and pass. (At least they used to--some of today's CGI block busters are exercises in contrivance). Not everything has to be based on a true story, but reality inspires thoughts of possibility--therefore you must be vigilantly observant and imaginative about what you experience.
As the old adage goes..."Write about what you know"...And then maybe throw a few imaginative twists in.
The 'best' approach is different for different people, and very much depends on what your aims are, and what your definition of good is. If you want to write something which will get artistic/critical 'acclaim' for doing something deep and significant then you don't want the same methods for doing a smart b-movie for example. And again for a shitty generic CGI bbuster. Personally I'm not a big fan of highbrow stuff but I would love to do a smart b-movie. Oh, and I guess in a way I disagree with the opening assertion - you can write something good if you decide to. But making the decision to do something good means doing what you have to do. You sure can't just sit down cold and tap it out (at least... damn it... I can't) And - haha - er - you do actually have to - DO - the writing - which I am currently struggling with. Shit.