FeeArrangementService.createFixedFeeMatterFeeArrangement(
   @feeArrangementRecordId,
   @matterRecordId,
   @matterPrice,
   @defaultForFees,
   @defaultForExpenses,
   @recurrence,
   @startDate,
   @endDate
);

@feeArrangementRecordId
The fee arrangement record's id value using which the fixed fee type of matter fee arrangement is to be created.

@matterRecordId
The matter record's id value for which the fixed fee type of matter fee arrangement is to be created.

@matterPrice
Your finalized matter price offered to client.

@defaultForFees
To mark this matter fee arrangement as the default when entering time for this matter.

@defaultForExpenses
To mark this matter fee arrangement as the default when entering expenses for this matter.

@recurrence
Provide how often the client will be charged for these fees.
(Accepted values are: One Time, Monthly, Quarterly, Annually)

@startDate
If your recurrence is other than One Time, then provide the start date for it.

@endDate
If your recurrence is other than One Time, then provide the end date for it.

The function will return a list of sObjects for the matter fee arrangement type.

advpm__Matter_Fee_Arrangement__c result = FeeArrangementService.createFixedFeeMatterFeeArrangement(
  'a0A9000000RJhKTE',
  'a05900000051CpN',
  2000,
  true,
  false,
  'Monthly',
  Date.valueOf('2023-01-01'),
  Date.valueOf('2023-10-31')
);
System.debug('Matter Fee Arrangement created : ' + result.Name);