I am always amazed at the huge number of visitors to my website's page on "Wearing Cowboy Boots" who find it by entering phrases like "how to wear cowboy boots" or "do you wear jeans inside or outside of cowboy boots" or "how to you wear jeans with cowboy boots" into a search engine.
Now, to be honest, most guys wear jeans over boots. Jeans are worn "stacked," meaning the jeans are long enough to fold softly across the foot and across the back of the boot, but not lower than the heel. More information and instructions are on my "Wearing Cowboy Boots" page.
However, if you have nice boots, you could consider wearing jeans inside them from time to time. My pictographic "jeans and cowboy boots" tutorial is visited at least 1,000 times each week. In observing the high frequency of these visits, I decided to update it by showing a simpler method of how to hold the jeans down inside cowboy boots. Come visit the page and see ... it's simple.
Life is short: wear your boots!