Writing should start with a plan. Whether you are working on a one-page essay, or a hundred-page thesis, it's good to build on a structure. Everyone has their own preferences, so you'll need to find what works for you. Maybe you procrastinate the content writing because the plan's just not perfect yet, or maybe you undercut your planning time because "nothing gets done".
A clear idea of your target audience and goal will help to make a good plan and structure. Bigger projects may need rough time planning to begin with, and with smaller ones you may want to go straight to the outline or structure of the text itself. Which subjects do you want to include? Which bits need emphasis? Do you have information that leads to one conclusion, or does your question have multiple possible outcomes?
If you're an overplanner, you might find it hard to make the leap from planning to writing. You'll know the plan isn't perfect, but you don't know how to improve it anymore. This can be a hard place to get stuck, but searching sources and actually starting to write can help pin down what's important and how you want to say it.
Remember, if you keep track of your subjects with subheadings, you can always change your order later on. You'll probably adjust the structure of the text as you write on and find out what needs more emphasis or explanation. Your plan is not set in stone, so don't worry about tweaking it as you go along.
Tight deadlines can trick you into thinking you must start writing immediately to make it in time. Starting without a structure can get you stuck though. It will be much harder to know how to start or where to go next. Having a backbone for your text allows you to work on the parts out of order, so it's easier to find a place to start when you sit down to work.
Secondly, to write without a plan risks certain information to be overlooked and to not be included. Making a plan will absolutely save you time in the end, and it'll help you deliver a more complete text.
How to put that structure in place
Start with noting your headings down and move their order until they roughly tell the story on their own. If you're following a standard headings structure, such as in an article, start with the figures and results. Once you have an oversight of your main findings and methods, work on your discussion and conclusion.
When you start on your discussion, make sure you keep track of your sources. Put the citation in when you first use the information, and add it to your reference list. (An automated list can save you a lot of work.) Use subheadings to keep an overview of your work and to judge its completeness. Ask yourself what background information the reader needs up front to understand your subject - maybe you can move parts to the introduction. Once you have the information under their right headings and subheadings, it's easier to change the order and add new information in the right place.
For essays and shorter projects planning should start with your main subject, findings or goal, and be built up from there (with a conclusion and introduction) in much the same manner as big projects. You can use headings for your own use, and decide which ones you need to keep in the end.
Specific projects, such as grant writing or the larger research process may have unique demands when it comes to planning you text or your time. For tips on research and article writing, check this free science writing course. It has a very helpful week dedicated to planning, and I highly recommend the course as a whole.
Even if you had to make an early version of your summary, make a new version once the rest of your writing is finished. You'll be able to capture the crucial aspects better, and put them down in a concise and clear way.
Sometimes you're far along in your writing, but you feel like somehow the text doesn't work as you intended, or you keep moving the same bits around and can't seem to get it right. Getting another set of eyes might be just what you need. Someone who doesn't know the content as deeply as you might recognize what is logical and what could be confusing to your readers. With a fresh look, it's much easier to see what should be shortened or expanded too. This doesn't have to cost them much time, and it'll save you a lot.
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