How I do it is since I don't b-stick (since I don't want to lose my smash attack speed), I put I index finger on the Y button and put my thumb on the B button. When I see that little sparkle coming out of the second jump, I immediately hit B while simultaneously hitting the analog in the opposite direction that you want to go (since you can get some wavebounce from this though not as much as with b-sticking). However, the timing of the Pk fire after the jumping can be difficult since you have to have only 1 frame between the start of the second jump and pk fire... It's mostly practice.
The second way is to do B-sticking by replacing your C-stick controls from Smash attacking to special moves. When doing this, you jump and use the same timing as before but instead hit the C-stick right after the second jump. This gives you m ore wavebounce from the pk fire (the reason why Lucas's B-stick) which will give more recoil so that you travel more horizontal distances.
You have to beware that in both style's of zap jumping, if you are holding the analog forward while jumping and then pressing B after, this gets rid of the wavebounce that you normally would receive and would cause you to Zap Jump forward. You could do this if you wanted to have the Zap Jump work that way for you but you can't Magnet Pull, a technique which gives you more horizontal recovery, afterwards if you do so (since Magnet pull requires the wavebounce of the PK fire).
That's basically what I can tell you about Zap Jumping. If anyone wants to correct me or explain this in a better fashion, go on ahead.
I haven't fully mastered this technique since when I get the stress of battling a player, I usually mess up...