How do I write off an invoice in Accounts Receivable as a bad debt?

The Direct Method - Each invoice that is a bad debt is posted to Bad Debt Expense (an expense account) as the bad debt is recognized.
The Allowance Method - A percentage of your Accounts Receivable is written off periodically or at the end of each fiscal year. The amount that is written off depends on the percentage of bad debt you believe your company incurs throughout the year. Normally, you would make a General Journal entry affecting an accounts receivable (used as a contra-asset) account titled Allowance for Doubtful Accounts and Bad Debt Expense (an expense account). Then each invoice is written off to Allowance for Doubtful Accounts as the bad debt is recognized.

Identify the customer and invoice number to be written off as bad debt.
Select Options, and then select Global.
In the Hide General Ledger Accounts section, clear Accounts Receivable (Quotes, Sales Orders, Proposals, Invoicing, Credit Memos, Receipts) box, and then click OK.
Click Tasks, and then click Receive Money.
Select the customer that has the invoice(s) that you wish to write off.
The Apply to Invoices tab will display with the invoices that are outstanding for this customer.
Enter a unique reference number which will help identify the receipt as a write-off, for example WO-001.
Place your cursor in the Cash Account field in the upper right-hand corner.
Select the Bad Debt Expense or Allowance for Doubtful Accounts from the drop-down list.
Select the invoice you wish to write off as a bad debt, and then select Pay box.
Click Save.
The entry will be reflected in the Cash Receipts Journal and the Customer Ledger.

Note: Once you have completed this transaction, be sure to change the Cash Account back to the appropriate GL account.