It's often tempting to shy away from formal letter writing as the language involved is so distinct from spoken English and it can seem like such a dry topic. However, writing formal emails and letters is one of the skills students are most likely to benefit from in the real world and it really doesn't need to be dull.
Here is a basic template which I give to my FCE and CAE students to help them approach the most common types of formal letter:
I usually give them the template before class and ask them to learn some of the phrases, then in class I share this powerpoint: Formal Letter Practice HighImpactEnglish.pptx
For online classes I give it to students to go through it in pairs, but it could equally be displayed on the classroom whiteboard.
The PowerPoint has a series of prompts for silly versions of the three principal formal letter types: Complaining, Appying, Requesting information.
In pairs, students predict/remember the exact phrases to begin each letter, then use prompts on subsequent slides to "say" (not write) the middle. After going through the four letters outlined in the PowerPoint, they should feel much more confident about the language they need in a formal letter or email.
Finally, set them a more conventional formal letter for homework to consolidate.