Reproducing the folds of cloth and clothing in drawn art can be a tricky endeavour: The material is inanimate, but changes form based on gravity and what portion is being held up by another entity. Jeans, the clothing for work and casual wear, present the opportunity to reproduce the same features with the added wrinkle that they come from the rips caused by wear and tear. Frayed rips can be created skilfully with an eye for gravity, folds and the individual threads that expose themselves when denim comes apart at the seams.
Draw the outline of the trousers --- two legs meeting at a unified crotch and hip --- in the position you see fit, whether that is fitted onto a body, or draped alone in the environment. If the trousers are on a body or on a piece of furniture, draw the other entity first and draw the trousers around it -- only give trousers shape where the object has shape, and draw the rest as a piece of cloth hanging by the force of gravity.
Draw a circle anywhere on the trousers that you would like a rip to appear, for example, a semicircle if the tear is on the edge of the trousers, the flat side being along the edge. Draw the structural details -- pockets, fly, belt loops and stitching -- onto the trousers, erasing and altering the outline where a detail like a side loop exists on the edge.
Under the upper and lower portions of each circle, draw jagged lines for torn fabric edges hanging by gravity. If the rip is on a portion of the jeans where they are stretched tight, such as around a character's buttocks, draw the lower jagged line very close to the circle bottom. If the rip is on a looser portion of the trousers, draw the jagged line farther downward. Connect the lines to the circle sides. Erase only the upper edge of the initial circle -- you want the lower edge of the rip to look like it is folding downward slightly. Draw a few stray threads on the rip, either horizontal across the rip or hanging broken from the sides.
Erase the lines you don't want to appear in the final drawing, such as the portions of a character's legs that were needed to properly draw the proportions, but wouldn't show under a pair of jeans.