Crafting Sigils?

I read that you can use 3 cores to craft a sigil in a topic related to Siralim 1 and then some mention of it in the Siralim 2 FAQ regarding sigils. I, however, cannot seem to find how to do this. I’ve been to the sigil modifier, checked the core and sigil menu, yet still can’t crack the code. Is it something related to rituals? Some other option that I’m missing?