I'm using the old-style Soylent pitcher with the big handle. I haven't tried the new shaker bottle.
The best way to stir Soylent into water is as follows:
Tighten the handle fully, since it is hard plastic on both sides. Tighten the stopper gently, so as not to warp the rubber seal. Hold it upside down and watch drip rate to test. Memorize the stopper position that minimizes drips.
To stir, hold the pitcher loosely at a 45 degree angle, in a relaxed comfortable position, dangling below the forearm, which should be 15-30 degrees below perpendicular to the floor. Hold the handle with thumb laying along the bridge touching the cylinder, to minimize shear stresses from handle to screw threads, which can allow leakage.
Gently and smoothly gyrate the handle so that the base also describes a circle, with a stationary point roughly at the pitcher's center of mass. This creates a vortex of water that mixes the Soylent sediment evenly. (Like the precession of the equinoxes.)
Pour with base elevated to ensure sediment reaches cup rather than accumulating at bottom.
Drink cup quickly before it settles.
Sometimes clumps still happen. If they float, I break them with chopsticks. For sinkers, I finish the pitcher, then break them up into the next one.