Fold up and measure half the amount you plan to shorten the pants. For me I'm hemming up 2" so I fold the pants up 1" (not including the original hem). Pin it so it don't move. You can also iron and pin.
Step 2: Line Up The Seams
Make sure you pay close attention to the stitches at the seam. If they don't line up you won't get a professional look.
Step 3: Thread Color
Most jeans use gold thread for the hem. Don't use gold. You want your thread to match the jeans. Your new hem will STILL use the gold or whatever color hem is already in your pants. My jeans are blue so I'm using blue thread in order to hide my stitches.
Step 4: Thread Color Cont.
I'm using the same blue in the bobbin. White would also work in this case.
Step 5: Sewing Machine Setup
You want to sew close to the original hem so select the according setting on your machine. Mine is B instead of A to get me closer.
Step 6: Starting
I start at the thickest part on a BACKWARDS direction then pass back forward. This is strongest. You might have to roll the machine by hand like I did to stop from breaking needles. Notice I run the foot along the original hem as a guide. It will keep you perfectly straight (as long as you pinned an measured correctly).
Step 7: Coming Up To The Other Thick Part
Make sure you line up the seams when they come into play well before they get close to the machine. This will keep you lined up. Again you might have to roll by hand at that point.
Step 8: Remove From Machine
After a complete pass remove from machine and trim and burn loose ends. Flip the seam inside (thus giving you the other 1" for a total of 2" hem) - that is why we only pinned half the amount we wanted to hem up.