Help with layer blends please.

So I’m just starting out learning UE4 and am finding it fun and annoying in equal parts but it’s only been a week lol. I downloaded a free pack from the marketplace for foliage and landscape material. The landscape material has 19 layers to choose from but I want to add my own but don’t know how to create the layer so it shows up in the options box. I’ve included a picture to help people understand what I mean.