Depth of field post process difficulties

Hello everyone, I am having some difficulties setting up Depth of field though post process since I am new to DOF and not sure how to get the correct settings for the map that I am working with. I have read the UE4 documents but I am still very lost on the set up. Basically what I want is to have somewhere around the mountains at view to become blurry based on the distance where I took the image. What options will I need to change for this I have been messing around with the DOF settings but I still can’t find the right settings to create the scenery I want. Can someone help me out with this? Thanks!