I'm trying to perform it from standing still (Stand still > Short Hop > DJ > Psi Magnet) and I only get Ness to travel a very short distance. Sometimes on rare occasions I have gotten him to travel farther.
If you want to cover more horizontal distance with your DJC, you'll need to make sure that you have *some* horizontal momentum mid-air when you input the double jump. If you're falling straight down when you input the DJ > Magnet you'll just get a slight upwards-forwards bounce. You don't actually need a whole bunch of momentum though, even holding forward for the single frame you input double jump on is enough to noticeably increase horizontal distance.
When doing the input for PSI Magnet, I should only move the control stick in one direction (down-slightly angled towards where I want to go), correct?
This depends on what exactly you're trying to do. The most "standard" way to do a DJC Magnet would be SH > DJ > Down(& slightly forward)B. Which will boost you forward without screwing with Ness's orientation. Other variations exist though, like SH > DJ > Down(& slightly backward)B, called a turnaround DJC magnet, which would boost you in the direction you were facing during the SH and DJ, but which would also swap the direction Ness is facing so that if the magnet connects you can more easily follow up with a Bair.
B-Reverse and Wavebounce DJC Magnet shenanigans are possible and useful, but they're not really as Bread n Butter as standard and turnaround DJCM. The vast majority of my personal B-Reversing is with non-DJC magnets to fake out an opponent trying to juggle or edgeguard me, or to keep my movement on/around platforms wacky, ambiguous, and evasive.
