Wear  regular shoes ,not your sneakers, and stand up straight. The height of the heel or the padding on top of a sneaker will change the amount of space on the front of the foot for the pant to not crinkle and to hang freely. Work from the SIDE of the foot not the front or back. You can measure by setting the end of a ruler on the floor. It should be an even distance from the floor. Put a straight pin horizontally to the desired length. Then take off the pant and continue it around with a few more pins without twisting the pant.

Test the length by using the pin line as your hem turning line. Gently press the new hemline and try it on again to be sure making necessary adjustments as you go. Trim the excess fabric and sew it closely to the original way it was hemmed by the  manufacturer. A great tool I use for measuring a is a chalk hem marker. And if all else fails, ask a friend to mark the hem or find a good tailor. Prices vary so be sure to ask the cost so there are no surprises and try it on before you leave. You don’t want to find there’s been an error when you are ready to wear them.