Landmark notes are given to learn reading sheet music effectively. Now there are a few different Landmark notes out there but I use the following 7 notes.
Middle C*… a universal name for this specific key
Treble G… This is a note which is indicated by treble clef which is on the 2nd line
Bass F… This is a note which is indicated by bass clef which is on the 4th line (2nd line from the top)
High C… This is a note 1 octave (next C) higher from Middle C
Low C… This is a note 1 octave (next C) lower from Middle C
High G… This is a note 1 octave (next G) higher from treble G
Low F… This is a note 1 octave (next F) lower from bass F
*I showed Middle C in each clef. Notice one in bass clef has a ledger line closer to the staff with bass clef (bottom staff) and other in treble clef has a ledger line close to the staff with treble clef (top staff)
Practice to find those notes until it comes instantly!