Use the LandscapeLayerBlend node. Select it and press the + to the right of “Layers” under “Material Expression Landscape Layer Blend” in the details panel (probably to the left).
Select the new layer you just created, press the arrow to expand and choose a layer name.
Do this for every layer you want in your landscape (for example Grass, Sand, Rock and so on).
Plug in the networks/nodes you want to affect each layer in the appropriate input that have appeared in your LandscapeLayerBlend node.
Plug the LandscapeLayerBlend into for example “Base Color” (or any other you’d like to use) and compile it.
Now select your landscape and see the “Landscape” section in the details panel. There you should find “Landscape Material” assign the material you just created there.
Now if you open up the “Paint” section in the landscape panel in “modes” you should see your new layers.