Mid-air horizontal sparks are the most tricky.
They can be performed by spin jumping, breaking spin, initiating the shinespark without holding jump, then pressing jump and forward.
One of the easiest methods is to hold forward the entire time, spin jump, release jump at the desired height, tap either up or the diagonal up angle button,
then tap or press jump again (while up/angle is no longer pressed). The timing of the jump after the up/angle is fairly lenient and doesn't need to be rushed.
Note that mid-air shinesparks can't be performed from a fall, only from a jump.
See also: Tech Page