The original how-to on the Dutch forum comes from the Dutch FZR forum, where the indicator was placed on a FZR600.
There are generally 3 different versions of gear indicators out there:
- The commercial tach/speed driven one
- The homemade "analog" one
- The homemade "digital" one
The downside to the commercial one is you need to tune it in, and when you pull in the clutch, the unit cannot calculate the right gear and displays nothing or even the wrong gear. The homemade ones connect directly to the shift cam assembly, so they always indicate the right gear.
I'm describing the "analog" homemade version, I don't have the schematics to the "digital" one, which uses an IC instead of diodes.
Wiring it to the bike
Regarding the connection to the shift cam assembly, you need (to make) the right cover plate (Neutral switch assembly):
The YZF750 from '94 does not have all the connections on the cover plate. If yours doesn't either, you have three options. 1) Quit, 2) Get one that has, 3) Make it.
Option 1: Buy the commercial Gear Indicator and be happy with it, they're not bad.
Option 2: The Thunderace seems to have a plate with all the connections in place (left in the picture), and it fits the YZF750. Partno for this should be 4FM-82540-01-00, NEUTRAL SWITCH ASS. Remember, the partno for the YZF750 for all years shows up as 1AE-82540-00-00, but the later models have more connections than the early ones (right one in the picture). Partnumbers and amount of connections are not confirmed yet, order at your own risk.
Option 3: Look for the marks where the shift cam assembly touches the cover, predrill a small hole and press a hot nail through it. Someone on the FZR forum did it.
He pressed the heads flush with the cover inside, cut the nails to size on the outside and soldered the leads to it. Keeps the oil inside and makes a perfect electrical connection to the outside. He mentioned it was a fiddly job and he'll never do it again, but that's not meant as a discouragement.
This is where the cover goes (the shift cam assembly is in the middle of the picture):
Some replace them without removing the water pump, I had no such luck.
Electrics
Now we have the connections to the gears in place, it's time for the electrics part of it. There are three main parts to the device, 1) Shift cam assembly cover connection, 2) Diode-block, 3) Display.
Bad quality schematic, and I plan to redraw it on some rainy day, but when you understand the workings of a diode, you'll grasp the concept of the workings of this gear indicator. The resistors are there to keep the LEDs in the display from burning through, you'll need one for each LED.
How you attach the switched 12 volt feed is up to you, as long as the display gets 12V.
Components used:
33 diodes 1N4007, 162272-06
01 display SA36-11, 158780-06
07 resistors 1Kohm 0.25W (0207), 403253-06
01 printboard SU527629, 530809-89
The dashed numbers are the partnumbers at http://www.conrad.nl, there are pictures to give you an idea of the parts.
Here's one of my builds:
Other guys have build different versions where the display is remote from the diode-block. One build the diode-block and display into the dials-assembly, others placed the diode-block near the ignition coils with a cable to the display he placed in the indicator section.
The cable I used was a CAT-6 ethernet network cable with solid cores (easier to solder). The guy who made the schematic used DIN-9 serial connectors to connect the diode-block-box to the "sensor" and remote display.
Display placement idea
However, in the dash I put 6 LEDs in a row and one single resistor. Simple and works. Having actual numbers is fancier, thanks for the post. 
