How to step by step eh... thats askin alot lol j/k
1. Buy/make tacho of your choice. I chose to assemble mine from a jaycar kit. See here for more details
http://www1.jaycar.com.au/productView.a ... &SUBCATID=
2. Fiddle with the settings of it (if its a bought one and can be adjusted)
3. Find a nice suitable space in your car, and make sure that you can reach some power and grounding circuits. I hooked mine up to my amp switch (lets to cut the amp off if cops are near

)
And yes i cheated with the reading, you'd be thinking i had major engine troubles with that many lights reading and the tacho at 3700 rpm lol.
4. Choose your own way of finding it, but feed the signal wire - green - through to the front of the car, personally i pulled the front-right grommit for the electrics to the power windows (if i had them) fed it thru there and up through the hole above my front-right fender.
5. Route the wire through the engine bay to near the battery.
6. Use a suitable connector to join onto the white wire., I'm using a joiner plug, tube of plastic with a crimpable centre, put the white wire (one bit to each end) and on the side closest to the distributer feed in the green wire, then CRIMP the fker.
NOTE! if you
do not attach this white wire, your car WILL NOT work.
And thats it really, should work... ah wait..
If you're connecting to the distributor wire and your tacho has 2 inputs (low level and high level [coil input]) use the correct one, so you dont blow your tacho up like i did, puzzled the jaycar dudes for a few hours trying to figure out what was wrong (i still dont know, i got home and changed a few resistors and twisted a variable resister for the dimmer thats built into it and it started working! woot.
Anyhow. thats it really, enjoy your new tacho.
PS. I advise that you go to an autoelectrician and ask him/her/it which wire is correct, it tends to change colours =P
Good luck!