When creating opening balances for employees the items that should be set before posting a payroll in Spire are the vacation owed, (which should total in the GL to the liability account set for accrued vacation) and the advances owing (which should total in the GL to the asset account set for accrued advances).
If the company is going live in any month except January first then they must also post a timecard to each employee for the total year to date from their old system. Before posting be sure to edit the taxes, deductions and benefits to match what was historically posted.
Optionally each timecard can be posted separately.