First you're going to choose a pair of pants that are either:
1. a little bit loose fitting in the butt, or
2. made of somewhat stretchy fabric (ie. jeans that are 99% denim, 1% spandex or something like that).
Next, turn them inside out and put them on. Take some pins and begin pinning either at the upper thigh or knee area all the way down to the bottoms of the pant legs. Remember to leave 1cm seam allowance! Now take them off and using your chalk, draw a line from the bottom of the pant leg to the crotch, over and around and back down the other leg. Use your pins as a guide and remember to check the widths of each leg with a ruler. Please note you are going to cut off the entirety of the original inseam, so draw a continuous line. (This is also why you must choose a pair of pants that are either made with stretchy fabric or are a little loose fitting... you're making your pants a little smaller in the bum, and you need to fit those buns back into your pants!!!)
Once you've drawn the line and you're happy with it, pin your pants on the line and try them on, one last time before you cut off the excess fabric. Remember, you can always redo sewing, you can't redo cutting!
Now cut off the excess fabric, using the drawn chalk line as your guide. Re-pin as you slowly cut away the excess fabric. Using a matching color thread, sew your pants back together, double stitching in the crotch area for durability.
Yay! Now you have straights/skinnies!