Step 1 - You’re going to import your mat functions and then you’re going to want to create two nodes: a landscape layer sample node and matblend simple node. As you can see in my photo, the matblend nodes allow you to plug up your material functions and then stack them on top of each other while also allowing you to plug in the sample node into the alpha.
Step 2 - The landscape sample node is going to allow you to import the splat maps as target layers. So you’re going to import the splat maps into the target layers, which will act as masks for the textures and also allow you to freely paint on the terrain if you wish.
Step 3 - Finish the network similarly to this. If you have a terrain normal map, you can import that as well and assign it to the normal map slot.
I know this was a rough explanation, but I plan on doing a tutorial series over the many different ways you can texture your terrains, so hopefully those will be better than this. Also, this is a very basic set up, so obviously you can expand on this however you like, I just wanted to help you get started with splat maps and blending.