First, pull the jean fabric in the back and bottom of the leg taught. You will wrap the fabric to the side of your ankle. Pulling it from the back and wrapping it from there will keep the seams of the jeans in line with the side seems of the Boot, making a neat appearance.
Next, I use an elastic band with hook-and-loop (Velcro® brand) fastening closure to hold the pants leg in place. I put it about 2" above the bottom of the pant leg. These elastic bands are called "boot blousers" and are available from Quartermaster Uniforms and almost any Army/Navy Surplus store. They are made to keep military boots inside BDU's but work fine for this purpose, as well as for wearing tall black boots with a uniform or leather if you pants do not have tapered legs.
Once I wrap and connect the boot blouser, I pull the pants leg fabric down toward my foot.
I then fold the fabric below the boot blouser above the blouser, and then pull the whole pant leg down again to make it taught. If done right, the boot blouser will be right below the ankle bone. (You don't want it right on top of the ankle bone, because it will rub inside the boot and may cause a sore.)
Pull the Boot on.
Tuck remaining loose pants leg fabric inside the boot.
Repeat with the other foot and Boot
Pull the Boot on.
And there you have it -- jeans inside Cowboy Boots, looking sharp!
Now, put on your western shirt and Stetson Hat, and you're ready to hit the town, Cowboy!
See more pictures of these Rudel Buttercup Cowboy Boots.