In the "slam for dummies" tutorial, laser scanner was used, and two methods of landmark extraction were shown.
But most practical SLAM implementations are based on camera images. In these applications how are landmarks extracted? The Durrant-Whyte paper does not mention it and I could not find an end-to-end demo project that includes landmark extraction.
Usually landmarks are many distinct, salient features like blobs or corners within an image/frame. Salient features in computer vision are an art in itself. There are dozens of different approaches to extract, describe and match such features (you obviously need to recognize them in different frames).
Wikipedia has an overview and here you see a video of a practical SLAM implementation running on a mobile phone (a little bit of advertising for an excellent university:).