Get relaxed. Take a break. Put on some music. Curl up on the couch for a while. Sit on your patio and listen to the birds chirping and the wind in the trees. If you prefer more active relaxation activities, go for a walk or a run. Work off the stress at the gym. Do what it takes for you to decompress before you face the page.
Free write. Sit at the computer keyboard or grab a pen and paper. Set a timer for 10 minutes and start writing. Write quickly, not paying attention to content, spelling or punctuation. Keep your hand or fingers moving, even if all you're writing is "I can't think of anything to write" over and over. Don't pause; don't stop. This is simply an exercise to get actual words from your brain to the paper (or monitor). When the timer goes off, stop writing.
Cluster. Brainstorm. Write your subject in the middle of the paper and circle it, and free-associate your topic. Draw a line from the middle circle, like a spoke on wheel, and write something else down and circle it. If that makes you think of something else, draw a line from that circle, write it down and circle it. If your topic is "new puppies," you might have circles for vaccinations, supplies, crate-training and food arrayed around the center circle. From vaccinations, you may have vets and cost branching off, and from supplies, you might have collar, leash, toys and carpet cleaner.
List or outline (if you are more linear-minded or analytical, you may prefer this method to clustering). Write your subject at the top of the page. Again, free-associate and write down ideas, leaving space in between to fill in what each subheading may inspire. Your "new puppies" outline may look like this:
New puppies
1. Vaccinations
a. vets
b. cost
2. Supplies
a. collar
b. leash
c. toys
d. carpet cleaner
3. Crate training
4. Food
Whichever technique you choose, the object is to get some ideas down.
Talk out what you want to write. Record this or have someone who's willing listen to you take notes. Many people feel more articulate as speakers than writers, and have no trouble rattling off exactly what they want to say.
Make a template or format, using headings and subheadings. If you made an outline, use that. Transcribe the recording or work from the notes, and plug the information into the appropriate section on your template. You will find yourself with a working draft and, perhaps, a little less writing anxiety.