As advertised, I'm not a trainer and don't claim to be, but this is stuff so
utterly basic that any eight-year-old ranch kid, some farriers, and even a
few equine practitioners can get it done.

Find yourself a good stout oak tree with a limb about eight feet off the
ground, preferably with sand or grass under the limb.  Tie a truck
innertube to the limb with several wraps of 1/2" soft lay nylon rope.

Fit your untieable horse with a brand new, doubled and stitched, nylon
halter, the kind with stainless fittings.  It should fit snug, but you
don't want it indenting the skin anywhere.  Have a passing boy scout or
handy cowboy plait one end of a 10' piece of 1/2" soft lay nylon rope
directly to the halter ring of your brand new halter.  Put the halter to the
horse, then tie the halter shank to the innertube with a bowline, with
about an arm's length of slack in the shank.

Walk away.

School is now in session.

Give a set of cymbals to the most energetic of the barn urchins.
Encourage the local motorcycle club to visit the stables.  Tell the barn
help to play norteño music as loudly as they do when  you're not there.
Whatever noises and whatnot Dobbin finds discomfiting are a Good
Thing.  Dobbin will discover he is no longer in control, he is tied.

Dobbin will pull on the rope:  the rope will pull on Dobbin.  The harder
Dobbin pulls on the rope, the harder the rope will pull on Dobbin.  The
rope won't break.  Neither will Dobbin's neck.  If Dobbin manages to get
a foot over the rope, he'll probably get upside down.  Leave him alone!
Let him lay and don't worry about it, he'll sort it out eventually and be the
wiser for the lesson.

After the initial fit throwing, hang a water bucket for Dobbin.  Don't
change anything in Dobbin's regimen:  do whatever it is you do with him
just as you usually do, the only difference being that he no longer
lives in a stall or paddock, he lives tied to a tree.

It's a pain in the butt to feed a tied horse grain, but it's easy to hang a hay
net and take it down when he finishes his hay.  Leave the water bucket.

Rain or shine, let him live a couple of weeks on the tree.  He won't be
able to lie down, but that's okay.  Before too long, he will come to accept
the proposition that no matter what happens, or what he does, he is
dominated by the rope.

This method is not designed to take into account any of the excuses
folks are prone to offer for the behavior of an ill-broke, common-broke
or unbroke horse.  It doesn't make any difference to the innertube if
Dobbin was abused, from a broken home, or has problems with authority -
Dobbin WILL stay tied, no matter what he does.  The innertube has
absolutely no regard for Dobbin's tender feelings, does not take into
account his inner child, and is totally passive in that it never pulls on
Dobbin unless Dobbin pulls first.  The innertube will win every
confrontation and, sooner or later, Dobbin will make the transition
from puke to broke in the tie department.