After reading this, if you see anything that is not correct, please comment. This is my understanding after reading countless articles and watching dozens of videos.
For this explanation I will tackle $1 and $2 notes printed on 32-subject sheets and overprinted by COPE. This includes $1 and $2 notes printed from the early 1970’s through the start of LEPE and 50-subject sheets. The sheets at this beginning stage are slightly oversized and will be trimmed later.
The first print is the back of the sheet. Green ink is applied through the intaglio printing process on full sheets from 32 plates mounted on a large revolving drum. The printed sheets move rapidly on a conveyer in order for the ink to skin-over, but the ink is not completely dry. These are left to completely dry. I’ve heard one day, and I’ve also heard several days. After drying, the sheets are inspected and flipped over to receive the second print.
The second print is where the front intaglio black ink is applied to the sheets. Included are the portrait, intricate border designs, the series date, denomination and signatures. This ends the intaglio printing process that gives our currency that distinctive feel. When leaving this stage, the sheets are cut in half (lengthwise) into 16-subjects sheets. I’m not certain whether there is another drying period before they receive the overprint.
The third print is where the 16-subject sheets receive the non-intaglio overprint on the face. This includes the green serial numbers and treasury seal, the black district seal and the four district indicators. The serial numbers are applied counting backwards in order for the lowest number to arrive at the top. This is where the sheets receive the distinctive embossing we love. The currency overprinting and processing equipment contains built-in electronic and photo-electric sensors that inspect the overprint. Stacks of 100 16-subject sheets then enter a cutting machine that trims off any selvage and cuts the sheets into individual stacks of 100 notes before they are banded. Forty packs of 100 notes are then shrink wrapped into bricks of 4,000 notes.