Okay it is in here...

https://neocoregames.com/en/community/warhammer-40k-inquisitor-martyr-compendium#section-season-of-the-void-brethren

Basically go into your currency tab and use your Void Brethren Insignia to create a Emp Seal.  Add that Emp Seal to a mission (in the tarot slots) and it turns it into a seasonal mission. 

Yeah, several of the new features are not very well explained in the game UI. I was scratching my head for a while, until I figured out you have to click the small checkbox in your currency tab. Clearly, you are supposed to read the mind of whoever designed this convoluted system.

Instead of the dropped insignia simply automatically increasing their count in the empyrean seals tab, you first have to open your currency tab, find the insignia section and then keep clicking the checkbox. If there ever was a pointless busywork in this game, it's this.

I appreciate Neocore's attempts to solve the problem with stacks of hundreds or thousands of tarot cards by creating the currency tab, but unlike tarot cards (where you need some degree of control where precisely you want to spend your tarot drops), there is no reason to process the insignias this way. They convert 1:1 to the seals, so all this can be done automatically, without the pointless intermediary of the currency tab.

Thank you

And maybe they also expected you'll watch the trailer, where they show the Seasonal Insignias right next to the previous empyrean and ordo seals.

I for one, like the fact that I can vendor the currency I don't want instead of them automatically converting to hundreds of insignias that I will probably never use.