How do you start an anarchy? You cannot start an anarchy; the anarchy is always there, just as you cannot create the cold, you can only remove the heat. Without people, there is anarchy, for the only rulers left of the universe are the various laws we use to explain why things react in which ways to what, and if we can define a ruler as someone with the free will to rule, we cannot accurately call those things rulers.
Rather, to return to the natural anarchy that began at the dawn of time, there would have to be no rulers--or, more accurately, no rulers by coercion, for, it is in my belief, there will always be voluntarily followed people of our world; parents who lead their children; the head of businesses who lead the people they pay for their time; teachers who lead their students; and so forth.
It's not about finding a plot of land somewhere, hoping and praying an invading nation stays out, because lets get real, nations always want more land, and if you wish to seastead, you must only wait for any government to stretch its law to include oceans. Space is more plausible, but trying to exist around other people who do believe in states will inevitably get difficult (for, even in anarchistic societies, there will always be some society somewhere which believes your anarchy needs to be conquered, especially if those people outnumber you); this is all assuming the people who generally believe government is vital and necessary to life outnumber the ones who don't.
Therefor, to return to anarchy, the people who do not believe in government must outnumber at least the largest government, if not all people who believe in the state. The revolution occurs first in the mind; the point is to change the minds of people, so they understand why anarchism is preferable. If you're under the belief that this is impossible, and people will always believe in government, and separatism is the only way; you may as well give up now, because, as all previous attempts at anarchistic societies have ended, it will only fail, and you will be assimilated into the greater group of people who
do believe in the state. (Refer to
this video for an example of the most recent anarchism attempt I know of.) Only until most people of this planet believe in voluntarism will we see a decline in government, and eventually, none; this, I believe, is the only way to achieve a life of anarchy. The sad truth is, it likely won't happen in our lifetimes, but perhaps our children, or their children, will have something better.