When you're in the client's project that you want to send a custom email to, click actions on the right side of the screen.
In the drop down, under General, it'll give you the option to "Send Email", click that!
Once you click "Send Email", you will be prompted with the screen where you will input your custom message.
The email template will already be set to "Custom Email Template", but if not, be sure to select it in the dropdown option.
Add the recipients email address, if it's not already in the system, you'll be given the option to "+Add" it.
Once you add the recipient's email address, enter your subject in the subject line and type out your custom email to the client.
If there are any attachments you'd like to add, feel free to do so by clicking "Upload a New Attachment".
Once done, click "Send"! You'll be all set!