Hello
Sounds like you’re almost there!
Left front neck edge is the left as you would be wearing the sweater. The 12 stitches are to be picked up and knit just from that edge.
If you hold the sweater up to you as though wearing it to get the left front, then flip it around so that you are looking at the front RS you will pick up beginning on the right (as you look at it) and work towards the centre front.
Either evenly space the pick ups by eye to get 12 (1 at the 1st stitch another 5 up to half way etc) or another option I used from a tutorial tip was to pick up in every stitch on the first row (the whole neck) and then adjust in the second row up or down by increasing or decreasing to the number the pattern states (12 on this part) the reason for this is to make the transition “full” and avoid little holes along the collar or neck band.
If you are still not sure where to pick up these 12 maybe post a photo.