In a nutshell, converting a PSD design to an HTML email template includes these steps:
Make the project directory on your computer and create two folders inside it (for images and CSS styles correspondingly).
Slice the PSD design into individual images in Photoshop or another tool and put those into the folder for images you created earlier.
Make an index.html file inside your project directory.
Create the template’s HTML structure in the index.html file.
Add links to the images you have sliced within the HTML code.
Create a CSS stylesheet inside the CSS folder.
Add the required CSS rules for the HTML elements to the CSS stylesheet.
Next step - detailed testing on different email clients, in different browsers, on multiple devices, and across major operating systems.