Clean off the braking surfaces on the rim then flip the bike upside down and spin the wheel. Take a sharpie marker and slowly move it towards the braking surface of the spinning wheel, the high spots will get marked first. Tighten the spokes on the side of the wheel oppossite of the marked surface ca. 1/4 turn. Repeat spinning/marking on the other brake surface, tighten spokes as needed. Clean off the original Sharpie marks and check again, if needed adjust the spokes again. This only works for wobble and not for out of round wheels.

P.S. A "Poor Man's" truing stand can be frabricated from a discarded fork and clamped into a vise. All you need to make one is the fork, some nuts and bolts and a few pieces of bar stock. (drill, file/grinder and hacksaw required for fabrication).