Buy a copy of DDR 2nd Mix - Extreme (Note: JP versions, they have to be the Japanese versions).

If you don't have that good luck, cause they're hard to find now.

Provided that the song you want to edit is on DDR Extreme (eg So Deep on DDRMAX).
If you're making this on a PS2 (Which also has to be able to run Japanese games) -

Make the edit on the PS2.
Save on the PS2 memory card.
Put a PS1 memory card in the other slot.
In the Systems memory card browser (If you turn the console on without a disc in the drive) transfer the edit data from the PS2 to the PS1 card. (PS2 Memory cards do not work in the card slots of a Arcade machine).

If you're making them on a PS1 it's just a simple task of saving it to your PS1 memory card.

Find a DDR Extreme ARC cab with memory card slots and away you go.

don't forget:

In the PS2 DDR library data, you need to access to the data options. you need to EXPORT the data file, before transfer from PS2 to PS1 memory cards.

the export data is next:

DDR SUPERNOVA 2 DATA [SONGDATA]

max 8 characters lenght.

once you've filled your memory card of edits, insert your memory card into the PS slots in the AC

you need to wait some time, more or less time, due to the edits and other data (like Arcade Link data)

IN DDR 5th MIX - EXTREME:

there is an icon simmilar a memory card in the song who has edit. press LEFT twice to access the edit data

DDR 4TH MIX/PLUS

you need to access "LINK MODE", pressing RIGHT twice. now select "EDIT DATA"

DDR 3RD MIX AND BELOW:

the edit data is loading in the song wheel. a memory card icon appears in the song. Press LEFT + RIGHT in the cabinet (not the pad arrows, the cabinet arrows instead.) and press START. you select the current edit data

---

some notes

each edit data consumes 1 block of your memory card. Arcade link for DDR 5th - Extreme consumes 2 blocks. Arcade link for 4th Mix Plus and below consumes 1 block.

you can transfer edit data between PS2 libraries. Just export and import.

you can store up to 30 edit data in a PS2 library data sheet. you need to create another sheet for keeping store data (takes about 150 kb of more space in your memory card)