Trail 421 starts approximately a 1/2 mile up the trail on Trail 419. Look for multiple rock cairns and the trail splitting off to the right near some slickrock. Follow the trail steeply at times up the side of a grassy hillside.
Eventually, the trail switchbacks steeply up to the top to join Scarp Ridge Trail. If ascending this trail, you'll be working hard! Don't be discouraged, however, because the views along Scarp Ridge Trail at the top are fantastic and some of the best in the valley.