Access the invoice you would like to send on the Bill Batches Tab or from the Invoices Tab in the Accounting Center on the Invoicing Screen. Click the Send Email link in the Command column for whichever invoice you would like to send. Your client will receive the email message from whatever email address you chose in the Outgoing Emails Settings tab. By default the email will go to the billing contact email. See Adding/Editing a Client topic for more information on updating the billing contact email.
When the email pops up, you can edit the subject and message you would like your client to see with the invoice attachment.
It would be a good idea to tell your client to add your email to their safe senders list in their email program. It would also be a good idea to check with your client for receipt the first time you use the EnterYourHours.com system to send him/her an invoice. The EnterYourHours.com System cannot control how receiving email servers will respond to the email and it could get blocked by an overzealous email administrator.