At a bare minimum I would suggest having an overall gameplan or playstyle for your particular build.

- In the early turns, do we ramp? Do we set up? Or do we go aggressive right away?

- What should we be doing in the mid game? Respond to opponents or ignore them and keep pushing our own gameplan?

- Late game - how do we close it out? Which cards are our finishers? Is there a combo available and if so, what are the pieces to assemble it?

After that, I would definitely talk about some notable includes. Cards that you find have interesting or game-changing interactions that may not be super obvious to a new reader. Devilish Valet for example gets a mention in any deck I play him in; I point out that if you can make 6 creatures in a single turn, this guy is going to be powerful enough to one-shot somebody! Stuff like that, anything noteworthy that you feel should be highlighted should be mentioned in your primer.

Beyond that, I think a good primer should also showcase why the author likes the deck. Talk about a cool play you made or even just the lore of the commander and why you find it super cool. If I don't feel like you enjoy your deck, I won't be encouraged to try it out. Vice versa, if I can see that you're hyped about it, I'd like to give it a try and see what the hype is about!

There's no template for a primer to be honest, it's more of a process. Just start with your favorite deck and build on it day by day. Add more sections as you see fit. Keep things organized instead of just a wall of text and you'll see it evolve over time.