Once the stallion is washed, and an erection achieved, a plastic collection bag is secured around the penis by tape (note, don't secure the tape directly to the penis, and don't have the tape so tight as to interrupt circulation - remember the penis will become engorged just prior to and during ejaculation), and hot compresses are applied initially to the glans of the penis, and once "thrusting" is seen, the upper portion of the shaft. The compresses we have used were of the thin (half-inch) man-made dishwashing "sponges" that are about 8 inches by 10 inches. These are submersed in hot water (55 degrees Celsius), and squeezed out immediately before application. The first is held in the "flat" of the left hand, with the flat of the hand being applied against the end of the glans of the penis. The stallion should then drop his hindquarters slightly, and start a thrusting motion against your hand. Allow the hand to move with the thrusting, and at the same time use the other (right) hand to form a "U" of the thumb and index finger, with the other compress inside the "U", and gently apply pressure with this hand on each side of the penis shaft at the upper end. This pressure simulates the pressure applied naturally by the labia, whereas the left hand is simulating the pressure applied when pushing against the cranial portion of the vagina/cervix. No great amount of pressure is required, and in fact stallions will often resent excessive pressures.
After a short period of this stimulation (it varies with stallion, but considerably less than a minute would be "normal") the stallion will hopefully ejaculate, and the semen is collected in the bag. If manual dexterity is one of your strong points, you can fractionate the ejaculate by "pinching off" the collection bag after a couple of ejaculatory spurts, thereby separating out the gel fraction, which is presented in the tail-fraction of the ejaculate.
Not all stallions will respond to this method, but many will. Collections can be carried out on the ground, or on a breeding phantom (or I suppose on a mare if one so desired). A good stallion handler is an important part of the team, especially the first few times this is done, as it is possible that the stallion unused to such handling may kick. Obviously for the same reason, suitable positioning of the collector is important for the same reason. One of the advantages of this method is that it can be successfully used many more times in the course of a day than standard collection techniques, and can therefore be used to "clean out" a stallion, or establish a depleted value prior to checking TDSO (total daily sperm output), in a shorter time period.