Gain a shinecharge, then while moving into an enemy near the top of a slope, frame perfectly switch directions while pressing jump and hold both back and jump for 4 or 5 frames.
Jump can be pressed one frame before or on the same frame as the direction switch, then after 4 or 5 frames release either jump, back or both simultaneously.
If only releasing back or jump then there is no time limit on how long the other input can be held, Samus will just remain in the slope spark pose until a shinespark is initiated.
See also: Tech Page