to issue an invoice of 21,111 yuan, it needs to be calculated on the pre-tax amount. The specific operation steps are as follows:

1. Calculate the pre-tax amount, and the pre-tax amount = invoiced amount ÷1.16

2. Fill in the invoice information. According to the requirements of the tax authorities, the invoice needs to be filled with information such as the taxpayer's name, taxpayer identification number, address and telephone number, account opening bank and account number, as well as the buyer's name, buyer's identification number, address and telephone number.

3. Issue an invoice, fill in the calculated pre-tax amount on the invoice, and fill in the relevant information according to the regulations. After the invoice is issued, it needs to be stamped with the taxpayer's seal or special seal for the invoice to ensure the authenticity and legality of the invoice.