Like easycleanwindows says, George will do one for you. But if you want to do it from a wordprocessor document all you need is:
An invoice number - maybe start at 10001, thereafter sequential
A tax date - the date of invoice
Your business name and address, if you are a sole trader then your own name as well with phone number
if VAT registered then that VAT number
The address of the business you are invoicing
a description of services offered - professional window cleaning
a total invoice value.
Terms of Payment...like payment due within 30 days of invoice date etc..whatever your trading terms are.
pm me if you want a template
Mark