From the main navigation bar click the plus icon.

Choose Record New Sales. The screen will look something like this ...

Next fill in the basic information in the top section. Start with the To field. Begin typing the customers name. After you have typed at least 3 letters, the auto complete dropdown will appear with matches. Keep typing to narrow the matches and select one when ready.

This will load the clients pending orders as well as qty information.

You can now add your items to the sale using the various add tools.

As you add the items to the sale the order qty will update.

When ready click the save icon which will create your invoice.

Heading back to the order detail view you will see the updated changes of the items delivered.