I was writing this up for a more definitive and detailed instruction. Just the parts list written so far, gathered after some adjustments through the years.
Sega/Agetec arcade stick for Dreamcast:
--parts needed--
(1)Sanwa or Seimitsu stick: Seimitsu LS-32 or LS-55-01 recommended
(6)Sanwa or Seimitsu buttons: Sanwa OBSF-30RG recommended
(1)Phillips screwdriver
(1)flathead screwdriver
(1)hex key set
(1)Dremel Multipro rotary tool OR metal file, cutting pliers, and screw punch
(1)15w soldering iron w/solder
(1)22 AWG stranded wire spool- 25 ft. or less
(1)6 inches of heatshrink tubing (sizing suitable for 22 AWG wire) OR electrical tape
(1)5-pin wire harness from original DC stick (use only the side that attaches to DC main PCB)
--optional parts--
(12)Seimitsu small one-touch connectors
(12)Seimitsu small metal terminals
(8)Seimitsu large one-touch connectors (used for LS-32, 40, 33 only)
(8)Seimitsu large metal terminals (used for LS-32, 40, 33 only)
(1)Sanwa JLF-H 5-pin wire harness (used for Sanwa JLF-TP-8xx only)
--optional parts for Supergun/universal conversion--
(1)DB-15 connector
(1)extra wire spool- 22 AWG stranded - 25 ft
(1)24mm button for 'coin', Sanwa OBSF-24 or Seimitsu PS-14-D/DN
The main thing you'll want to note is the table at the bottom. He matches which color matches which direction. and which color on the sanwa harness you should match it to. Ottherwise, it's pretty easy. Even I did it, and I suck at this shit.