playing in position pt. 1: don’t get lost

Still on hiatus from natural harmonics, and having proposed that the purpose of playing in position is not to get lost, let’s look at how this works in practice.

An element often neglected in those fingerboard diagrams is any indication of each finger’s function in position playing; in particular, the function of the first finger. The first finger is responsible for your bearings.
Let me say that with some emphasis:

In position playing the guitar player gets their bearing from their first finger.

Looking at piano, as a contrasting example, the pianist gets their bearing from the position of their thumbs. Thus, the pianist will shift their hand, aim to land their thumb on the appropriate key, and, while moving into position, the hand shape will be altered depending on the line/chord shape. A similar principle applies to the guitar and guitarist, except the player gets their bearings from their first finger.
One of the simplest ways to practice this is to play a line on a single string. You can do scales, arpeggios, or, my preferred pedagogical atom, a melody. Take something you’re familiar with; something with a mixture of intervals. Something like Somewhere Over the Rainbow or Morricone’s music from Cinema Paradiso:
Relax. Find a comfortable posture. Move your fingerboard hand into position. Choose, arbitrarily, one string. Stop the string only using the first finger and play.
Try and feel the movement of your body as you do this. Mentally note how each interval leap, each part of the fingerboard, feels different in your arm, shoulder, in your body. (If you’ve ever had problems playing with your eyes shut, this may be the time to try closing them again.)
Experiment with the motion of your arm. Try smooth motions, abrupt shifts; try slides, glissandi or play the notes staccato.
Try different phrasing; try different strings; try different melodies.
Try improvising on one string with the first finger stopping the string.
Have fun (but don’t over do these).

Attention: I have no knowledge of your physiology, your particular instrument, or your posture. Please, please, please, note that the following may not be applicable to your situations, and, at best, should be adapted to fit your your needs. Be careful and listen to your body: If it hurts, or it’s uncomfortable, stop and review.
Although I cannot give explicit advice about movement without making assumptions about your posture, and, consequently, your instrument and your body, keep in mind that position playing is “a framework for linking together small and large gestures”, and thus what we are practicing here is the macroscopic gestures. Generally speaking (and always adapt this advice based on your real-world posture, etc) the first finger’s movement should be macroscopic. Do not be afraid of broad, sweeping gestures. The movement should not come from the twist of your wrist. Try moving your arm, swing your elbow, work your shoulders, etc. You may feel your back, buttocks, legs affected by, or contributing, to this movement.
If your movement is hampered (e.g. your elbow is locked into your ribs), you may seriously want to consider reexamining your posture.