Can someone explain where exactly to install O-rings or some other part to lock gear in LOW. I understand how to flip the gear so you are locked in high but where are you putting these rings to lock it in low.

how drill transmission shift gears and how O-ring work
 

The way the drill transmission shift gear is that the metal lever on the transmission slid a ring gear inside the gearbox to either:

Lock the second stage of planetary gear with the first stage so that the transmission skip one stage of planetary gears.

Or, it unlock the first and second planetary stages and engage the second stage to have extra reduction.

The O-ring act as a spacer to prevent that sliding ring gear from moving from one side to another, holding it to lock the first and second stage together, OR holding it to engage the second stage.

You will have to open up the gearbox to put the O-ring in. Just decide between high and low gear.

If you want to keep it in high gear, the lever is supposed to push the ring gear TOWARD the motor, so you can install the O-ring next to output shaft-pointing face of the ring gear.

If you want to keep it in low gear, the lever is supposed to push the ring gear AWAY from the motor, so you can install the O-ring next to motor-pointing face of the ring gear.