Alrighty, here are some options. The regulation states (from AR 670-1) that they must present a neat and a non-"peg-legged" appearance. What's that mean? There are tons of options. Some popular ones are:
1 - Using the green cords. Placing them over your boots, tucking the pant over the outside of the cord and under the inside so that the pant (and cord) sits on top of the boot instead of the leg. This is VERY comfortable for day to day wear, but do not recommend it for the field.
2 - Using the blousing bands (velcro bands). There's a couple of ways to use these. Before putting your boots on, pull the pant leg down, folding it etc., to the desired appearance and applying the band on top, below the boot line. Put your boots on and the degree of tuck/sag in the pants on top of the boot is up to you. Better for the field.
3 - A variation on # 2, and it's what I use at formal events in BDU's (i.e. change of responsibility/command ceremonies) is using the blousing bands, and cuffs. Tuck the cuff of the pants up, and apply blousing bands. Place blousing cuffs on the inside of the pants and allow them to form a crisp line at the base, over the boot. Pull your pants up and you're done. The appearance is a perfectly level line about 2 eyelets below the top of the boot and very comfortable, especially with highly pressed BDU's.
In all cases if you want to sage the BDU pants over the top of the boot, do not go below more than 3 eyelets to stay within regulations. Not being in the Army, this won't apply to you, but it's nice.
Ultimately, how the pants are "tucked in" is up to you. There are as many ways to do it as there are ways to shine boots. It comes down to what you need to do. When I was at Airborne School, we could not have pants bloused with cords or bands. They MUST be tucked into the boots. Too easy. A nice fold and you're done. Even with the starch washed out of them (BAC rules) they still looked high speed. Try a couple of different ways and IM with any questions.
SPC Richard A. White, Senior Medic
249th MP Detachment (EACF)
Camp Humphreys, ROK