The easiest way to enter the general ledger opening balance is to enter it as one large journal entry. You may wish to include a memo such as “Starting Open Balance” to identify this. In order to be able to post opening balances to control accounts, Company Settings must have the GL disabled.
The date to use for the GL opening balance is the closing date of the the old accounting system.
Another 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 determining if creating a spreadsheet with the correct layout will be more or less work than manually entering them. The spreadsheet layout is critical and the import will fail if not correct.