For any written work writing an outline is very useful. Write down all the chapters, sections and subsections and in them gather bullet points of the content und key messages, e.g. like this:
1. Literature review
2. Scope of work
3. Importance in scientific context
1. Some experimental methods
1. Some data
and so on and so forth. The more specific you are, the better. Write down any idea you have now, it might be gone later. If you think of a sentence that sounds good to you, but is somewhere in the middle of your paper, write it down! It can be the seed for a much longer paragraph
I usually like to get literature review out of the way first, because it is the most tedious. Gather the sources you want to cite and construct a nice introduction around them.
From here on it is just writing, no way around it. Personally, I write rough drafts of every section and come back to it later in an iterative process. Some people may like to perfect one section first before going to the other.
If you have trouble concentrating in front of the screen, get up and write somewhere else using pen and paper. Print your rough drafts and write your corrections on them. I am usually more focussed when reading on paper.
I hope some of these tips help. This is not only restricted to papers, but applies basically to any scientific work, theses, reports, etc.