The following is an example of a basic invoice, and how it could be formatted. There are a few places in which you could place the invoice number and date, but here is an example:
(this is you)
Emma Zerner
address/phone number
(this is them)
BUSINESS/CUSTOMER
Address/ATTN: PERSON, Email, etc.
Date of Invoice/Invoice Number
Services Performed:
Ate Donuts: $3.00
Ate Chocolate: $4.00
Total Amount Due: $7.00
To be paid within two weeks within receipt of invoice. You can restructure the format, and elaborate what best suits your needs, and what is relevant to the task performed.
Great Services
You can use Microsoft word or other word processor to complete your invoice. There are also some services online available to make your job easier, such as www.billmyclients.com. This company has a tutorial to walk you through the process and explain what you are doing, in addition to having templates, so your data is put in a professional spots, and you don't leave anything important out.