Great putting is all about distance control. Most golfers have a generally decent sense of the break of a putt, whether long or short, but distance control in putting is the key.
I use two drills to work on lag putting:
To emphasize distance over being “hole bound,” I have my students putt their lag putts from the green out toward the edge and have them stop the ball as close to the edge as possible.
I also use a ladder drill where I put four tees in the ground in a row 2’ apart and have my student putt to the farthest tee first, gradually working their way back 8’ to the nearest tee.
Mechanically, lag putting stroke quality has to do with the lack of “suddenness” in the stroke. Developing a metronome-like stroke on long putts is the most important mechanic.
Finally, for those who have very little body flexibility, I work with them to soften their hands through impact as opposed to the stroke being dominated by the shoulders, which is the case in putts inside 20’ on most greens.