Donald Trump's motorcade has arrived at the New York State Supreme Courthouse in lower Manhattan.

Court is scheduled to get underway at 10 a.m. ET, with Trump scheduled as the day's only witness.

Donald Trump, in New York, had a prep session with his lawyers Sunday ahead his testimony, sources tell ABC News.

Trump's mood during the session fluctuated between fits of anger regarding the case and being "in a good head space," according to sources.

The former president "can be a good witness if he stays focused," one source said.

Former President Trump is due in court to begin his testimony in his $250 million civil fraud trial at 10 a.m. ET.

Hundreds of members of the news media are huddled outside the New York State Supreme Courthouse ahead of Trump's arrival, with some having waited overnight.

After multiple in-court disputes about communications between him and his law clerk, Judge Arthur Engoron modified his limited gag order to cover attorneys in former President Donald Trump's civil fraud trial.

"Defendants may reference my staff as is appropriate to ask about scheduling issues or the management of the trial, which is an integral part of their jobs. What they may not do is to make any further statements about internal and confidential communications (be it conversations, note passing, or anything similar) between me and my staff," Engoron wrote in his supplemental limited gag order on Friday.

Engoron wrote that defense lawyers Chris Kise, Alina Habba, and Clifford Robert made "repeated, inappropriate remarks about my Principal Law Clerk, falsely accusing her of bias against them and of improperly influencing the ongoing bench trial."

The attorneys have raised multiple arguments during the trial that Engoron and his clerk passing notes between each other suggests impropriety and is distracting. Going forward, if the lawyers want to object to communications with his clerk, Engoron advised that they refer to the order as a "blanket statement."

"This gag order is as narrowly tailored as possible to accomplish its purpose, which is to protect the safety of my staff and promote the orderly progression of this trial," Engoron said.

To justify the safety threat, Engoron added that his chambers has received "hundreds of harassing and threatening phone calls, voicemails, emails, letters, and packages" since the start of the trial,

The judge threatened "serious sanctions" for violations of the extended order.

Taking the witness stand as an expert witness for the defense, accountant Jason Flemmons offered testimony in support of Donald Trump's approach to valuing his Mar-a-Lago property, which has been the subject of debate throughout the seven weeks of the trial.

In his summary judgment decision, Judge Engoron found that Trump overvalued the estate by at least 2,300% because the Palm Beach County Assessor appraised the property's market value between $18 and $27.6 million after Trump signed a deed that restricted its use to a social club, potentially limiting its resale value as a residence but ensuring a tax cut. Trump, in contrast, listed its value in his financial statement between $426 million and $612 million, and during his appearances in court and online he has repeatedly attacked Engoron's finding.

Flemmons argued that Trump's approach to valuing his assets gave him latitude to consider his property's future revenue streams. That approach, according to Flemmons, could result in "wildly different values" between the numbers listed on a personal financial statement and a tax assessed value.

"Tax assessed values are typically on the lower end of the spectrum," Flemmons said, while Engoron looked on attentively.

While he never mentioned Mar-a-Lago by name, Flemmons was asked by defense attorney Jesus Suarez about a hypothetical property assessed at $18 million but valued closer to $500 million using a comparable sales approach -- the same approach used to value Mar-a-Lago.

"It would not be unusual to have a value in the hundreds of million using projected cash receipts," Flemmons said.

Engoron then turned his chair toward Flemmons and began asking his own questions.

"I am trying to get to the order of magnitude we are talking about here," Engoron said. "What is the highest value you have ever seen legitimately placed on such a property?"

Flemmons could not provide a specific example to answer Engoron's question but reiterated that a massive discrepancy could be appropriate.