I usually work alone, but when I install upper cabinets, I always enlist the help of two shop-made cabinet jacks. They’re steadier than an extra pair of hands. The jacks stand on the lower cabinets. To position and level the upper cabinets, I just turn two bolts in each jack.
The jacks are 15″ tall and 12″ deep. Make them from 2x4s, with 2×6 bases. In the jack’s top members, pound two 1/2″ x 2″ coupler nuts into tight-fitting holes. Thread a 1/2″ x 6″ full-thread hex-head bolt through each coupler nut and cap it with a rubber chair-leg tip. –Boyd Card