To create a custom email template in HubSpot, simply navigate to the marketing email tool, select the "templates" tab, and click "create new template." From there, you can choose to start from scratch, import an existing HTML file, or choose from one of HubSpot's pre-built templates. Once you're in the template builder, you can add and edit modules, change colors and fonts, and adjust the layout until you have the perfect design for your email. When you're ready to send, simply select your template and start drafting your message.