Get started by doing something for just a minute or two, and then get up to applaud yourself for getting started.
If you have to write something, just write a sentence. Then get up, get some water, stretch. Pat yourself on the back for getting started! Now do a little more: write a few more sentences. Get up, take a mental break (don’t go to another website), do a few pushups. Go back, do a bit more. Pretty soon, you’re in the flow of it.