If Trump can't manage to write a formal letter to another head of state with more polish than the sub-literate rambling and disrespect on display here, then it's a disgrace. He wasn't talking off-the-cuff; he had plenty of time to look this over, edit it, review it, see if the tone fit the circumstances, and run it by other people to help out.
This brings up two questions:
a) Should have written the letter in "standard western diplo-speak"?
b) Would writting the letter in "standard western diplo-speak" get him closer to the desired result than what he did instead?
Disrespect? Not sure if you know that, but politicians behind the scenes do not discuss things in the kind of formal language you and the media expected to see here. Maybye some western politicians do, and vast majority of them do it in public, but in most other countries, it's a completely different matter. In Poland there was a bunch of leaks of private discussions between politicians, and no matter which part of the political spectrum the leaks were from, they don't talk with the same style they do in TV.
Moreover, it's Erdogan we are talking about. My personal opinion would be that he does not respect the kind of people who communicate by "standard western diplo-speak", and why would he, considering how often he runs roughshod over such people and gets away with it (see: EU).
As for how a withdrawal should have been done, for the first part a decision should have been made months ago and not when Trump was on the phone with Erdogan. This had in fact happened before which gives the DoD and State precisely zero excuse for being surprised since it was clear Trump wanted to leave but they were in fact surprised.
That just means that they weren't truly surprised. They were pretend-surprised for public effect. "We were really hoping he stops making waves about the matter and defaults to our opinion instead, but oh well" kind of surprised.
Once the decision had been made months ago it should have been directly communicated to the commanders in charge of operations, and on the ground, and a plan for a safe and smooth withdrawal should have been drawn up.
Maybye it was, in which case we don't know because that was classified. Maybye it wasn't, and in that case, there would be a half decent excuse - we all know how leaky the administration is.
The YPG/SDF should have at that time also been told that the US did not sign up to fight Turkey for them, that we would be leaving in the near future, and that they should make whatever arrangements they needed to.
Does the lack of continuing defense treaty even require stating out?
Also even if the administration wouldn't leak, scorned YPG would leak for sure because it would have been their only chance to get the establishment "hawks" to have time to pressure the administration into staying there.
Consider what happened the last time Trump wanted to disengage from Syria:
Trump declared victory over the Islamic State in Syria, “my only reason for being there during the Trump Presidency,” he said. Key senators blast move
eu.usatoday.com
That was nearly a year ago...
Sen. Lindsey Graham, R-S.C., derided Trump’s decision to withdraw, likening it to those made by former President Barack Obama to announce ahead of time plans to reduce forces in Iraq and Afghanistan.
Guess Trump listened to Graham this time, at least to the point about announcements...
If announcements did not bring such political counter-interventions, there would have been nothing to withdraw by now. Also yet another reason why all the people who act surprised are either really naive, ignorant or dishonest.
They in fact had been negotiating with Assad and the Russians last year and broke it off based on assurances from State and the military that we wouldn't leave them in the ditch; then we proceeded to leave them in the ditch.
From what i remember from that time, news stories broke out that they were negotiating with them with rather unreasonable demands.
If they have interpreted "wouldn't leave them in the ditch" as "will commit to indefinite military presence in Syria, come hell or high water" that's their problem of hearing what they really, really wanted to hear rather than what was said.
So there should have been an agreement ready in the wings to bring the SDF under the umbrella of the Russians and Syrians, probably with a better deal than they got here because they'd have more time and leverage.
The situation and negotiating positions were shifting all the time. The details of whether Kurds got the most optimal deal possible out of Assad are not a US responsibility, worry or problem, it's Kurd's. Last year they weren't too eager to make deals with Assad, hence their high demands, but their negotiating position was better then and everyone knew that it sure as hell isn't going to get stronger as Assad finishes off the war and consolidates his forces, it was bound to only go down.
US troops should have already been moving out and Russian and Syrian forces moving up to replace them opposite the Turks before the withdrawal was announced. Erdogan should have been surprised by this, not the SDF and American commanders and diplomats. Instead Trump impulsively made a decision and announced it on Twitter right away without giving anyone time to prepare, except that is for the Turks.
Everyone else should have expected this just as much as they had the same knowledge to go on as Erdogan did.