Many surfaces need to be "cut" to make it possible to unwrap them. We call the edges that will be cut "Seams". In Edit Mode, selected edges can be marked or cleared as seams, using Mesh -> Edges -> Mark Seam (Ctrl E). Here you can see a cube with seams, and the resulting UV map after applying LSCM.

Often a mesh cannot be unwrapped as one group of faces, but must be cut up into multiple groups. If seams divide the selected faces into multiple face groups, then LSCM unwrapping will unwrap them separately, and position them in the UV editor so the face groups don't overlap. To ease selection of face groups, Select Linked in UV Face Select Mode Select -> Select Linked UVs (L Key) will now select all linked faces, if no seam divides them. This way, you can select a face group by selecting one face of the group, and using Select Linked.

To further tweak the result, UV' in the UV editor can be pinned to a fixed position. If LSCM is executed, these UVs will stay in place, and the resulting UV map will adapt to the pinned UV's. In the UV editor, selected UVs will be pinned or unpinned by using UVs -> Pin (P Key) or UVs -> Unpin (Alt P). Using UVs -> LSCM Unwrap (E Key) in the UV editor will launch LSCM unwrapping on the faces visible in the UV editor. Pinned UVs are marked in red.

Drawing of Seams in Edit Mode and UV Face Select Mode can be toggled with "Draw Seams". Other new drawing options in UV Face Select Mode are "Draw Edges" and "Draw Hidden Edges". The colors of seams and these edges can be changed in the Themes options. "Draw Faces" in UV Face Select Mode will draw selected faces in a transparent purple (or the theme color), similar to Edit Mode.

Demo images below by Derek 'BgDM' Marsh: