Use this simple fix if your salwar is a good length, but the cuffs slip under your heels while you're walking.
This problem is usually related to the cuff width. A cuff that is too narrow will scrunch up around your ankle. A cuff that is too wide can fall over your foot and slip under your heel or sole as you walk.
To fix cuffs that slip under your heel, make the salwar cuff narrower. Turn the salwar inside out and smooth the fabric of the leg flat, with the seam at one side. Using a needle and thread, stitch through both layers of fabric to create a new seam as shown in the diagram below:
How much narrower does the cuff need to be? Take the cuff width from a similar-style salwar that doesn't have the same problem. You can also put a safety pin in the cuff about an inch in from the seam, check the result, and move the safety pin as needed. For most communities, the cuff should be wide enough to puddle a bit on the top of the foot and fall partway down the heel, but not wide enough to fall off your foot and slip under your heel. Check the salwar length in the mirror to make sure the cuff falls right. Hemlines are rising a bit in the Bollywood scene, but many traditional communities still consider it immodest to show your ankles.
Once you've figured out where the new seam should go, start stitching at the bottom edge of the cuff and angle your seam line to join the existing seam smoothly somewhere 5-10" above the cuff. Next, turn the leg right side out and press the seam flat. If you notice any odd jags or puckers along the seam, you might need to run another short seam to smooth out any zigzags or the angle where the new seam joins the existing seam.
Fashion has dictated narrower or wider salwar cuffs over the years. Currently (2019), cuffs are getting a bit narrower for traditional salwar, but are going wide to the extreme in palazzo and shahara styles as they join the fashion mainstream.