In life it often pays to be prepared. For example, before going for a night out we check that we have enough cash to avoid embarrassment when trying to get drinks. This same level of preparation and preparedness is also essential when you are writing a manuscript for submission to a journal.
Each journal is different and may have different rules on how much you can write and how to structure your manuscript. This might limit the number of figures or references you can include, or request specific details be included in subsections. You may think that you can edit your manuscript to meet these guidelines at a late stage, but this can be hard. If you have cited 100 references and are allowed a maximum of 30 which should you cull?
Maybe you think that you do not need to abide by these rules and your work will be accepted whatever. I have found that this is often not the case. For example, if you submit a manuscript that is overly long the editor will send it back with a request to edit the word count down and resubmit. This is additional work and time that could be better spent moving the research forward or writing grant applications.
I would, therefore, advise you to select your target journal, having written an outline and before you write a first draft. This enables unconstrained creativity when developing the initial flow of the article and deciding what to include, but then allows a targeted approach after this, which can maximize your chance of being considered by your chosen journal. By selecting the journal after the outline you also have more information at your disposal about the proposed manuscript and can reject journals if they do not allow a particular aspect of your manuscript, possibly seven huge tables or a video.
So be prepared and increase your chances of success!