Well, there are two tools that you need:
First is some kind of a miter box. You have to buy or make up a fixture which clamps the neck securely, and has a pair of guides that controls the path of the saw, and makes sure that it stays perpendicular to the centerline of the neck. There are many ways of making one, but it needs to be stable and repeatable.
Second is an accurate measuring tool to locate the positions of the frets. The simple thing is to buy a Machinist's Rule, which is a very accurate ruler. You need one 24" long to do most bass scales. They come in different types of graduations. My favorite has fractional inches on one side, and decimal 10ths and 100ths of an inch on the other.
Here's the McMasters page for Machinist's Rules:
http://www.mcmaster.com/#machinists'-rules/=k31tzc
A top-grade Starrett 24" rule (2285A8) is $95. An import alternative (2042A79) is $34.
Careful measuring and layout using one of these rules will locate your slots to within 0.01". That's generally okay for hobby work, but we like to get higher accuracy than that.
In my own case, I bought a surplus vernier height gauge. This is a precision measuring tool that can read to 0.001" over a 40" range. One of these costs about $2000 new, but I got mine at a used machinery place for $100. I modified it into a special frame, so that I can use it to make up my own custom scale rulers of any scale length I want. Those custom scales fit into a special powered fret saw that I built. It's a lot of work, but I make my living building custom necks and basses. That's how I approached the problem.
You can also have a local machine shop make up a custom scale length template for you. If you provide the chart of numbers, they can cut a series of notches or slots into an aluminum bar for you, to an accuracy of 0.001".
I get my numbers from an Excel spreadsheet that I made up about 20 years ago, using the standard formulas.
