You really can’t suggest any easy process or as a standardized process to compose song, I mean this is something which is kinda so subjective in nature at the same time subtle and simple….
Songwriters write songs , so songwriting is writing song , so if you want to be a song writer you gotta have a song in mind and a good visualization of what you are you going to do with the song, because the band or singer , producers , engineers will ask about how you want to design the structure, the lyrical themes and many more in depths concepts and you better have a good answer for all of em.
Anyways writing a song is all about feeling it and going with the flow, see music is a very personalized experience and same is when you write a song it is more about the vibes and the feel that hooks up with you and how you want to manifest them acoustically.
So it totally depends whether you want to write a melody first, or you want to start with a cool groove to fill the low ends and low mids , the dynamics and manifestation totally depends on weather you want to layer a melody , bass or rhythm to the drums or any other possible way.
Production and songwriting are creative fields and hence will require a lot of creative juice.
For me song writing is very intuitive , I feel , I jam and then record but a lot of Melodies comes into my mind when I do a lot of scales up and down runs.
So based on your level of musicality choose the approach , the easy way is to see interviews of your influences and then try to understand their creative and production process and then execute those set of information to get the right sound, dynamics, structure and theme you want.
See as I told you already music production is a darn subjective process and I can’t sum it all up in this article but as a matter of fact, I will keep on sharing lots of articles on the same freaking domain.