Publishing in a journal is quite different from writing your PhD, mainly because you are restricted in terms of structure, word count etc.
The best place to start is to pick which journal you are going to submit to BEFORE you start writing anything. This is important as each journal has different rules so it is really important that right from the start you know exactly what format and word count you must stick to. Even little things like how they like you to write units can be a pain to change at the end if you suddenly discover yours are all in the wrong format.
In addition, once you have selected your target journal then I recommend reading a few examples of papers they have published recently, so that you can get a feel for how they are written in terms of language, flow etc.
If you google 'writing for an academic journal' there are lots of useful guides out there from various universities that should help.