Tears and rips in jeans just give them more personality and a flair of you not giving a rats tail. But when they just don't happen on their own to a very plain pair of old jeans you can give them a little help with a couple tools and 10 minutes.
The best tools to rip your jeans are scalpel/exacto knife, and a sheet of sand paper. It will be the quickest and easiest way to do it.
I would first put the jeans on and mark out where you want the tears to go
Then just take your scalpel/exacto knife and slice where you've marked
Now take the sandpaper and sand down the edges of the tears to give the fibers more softness
I find that when you also put them through the wash the tears will become more weathered looking