The easiest way to enter the general ledger opening balance is to enter it as one large journal entry. The memo can be entered as something like “Starting Open Balance”. To accomplish this the user must first turn off the GL, otherwise they will not be able to post to the control accounts.
The date to use for the GL opening balance is the closing date of the the old accounting system.
The other way to post the opening balance is to import it as journal entry which in the end has the same result as entering it manually.
Decide which method to use by figuring out if creating a spreadsheet with the correct layout will be less work than manually entering them. The spreadsheet layout is critical and the import will fail if not correct.