Hello,
I just follow this tutorial for textrues that is the best way. I saw that the landscape demo als uses this.
This part can be confuseing, to short it a bit down. First export the spat maps als hight output and then select PNG. Make the material show in this tutorial. then look at this picture.
You will se that and then you right click on one of those layers/elements, Then you will see a option for inporting inport that PNG splatmap. there you go.
And a tip you can use more than 16 texture’s tou need to search it. you you can make a massive material. and you use that as landscape material.