How exact do fret slots have to be cut?

[curtisa]

48 minutes ago, PRSpoggers said:

But for my first time is using a regular miter box fine?

In theory, yes. Provided the slots you have cut are perpendicular to the centre of your fret board blank and are positioned to an acceptable degree of accuracy as derived from your notched template, you should be fine. My suggestions above are only there for you to try out which may help improve the accuracy of your next board. The Rolls Royce solution is to use the template in the manner for which it was originally intended (ie, with the matching Stewmac mitre box), but that's not to say it is the only way to use it and still get acceptable results.

[Bizman62]

4 hours ago, PRSpoggers said:

But for my first time is using a regular miter box fine?

If you can add a pin to match the notches then any mitre box will do - more or less. Many people have built the mitre box by themselves. The main thing is to keep the saw blade upright and perpendicular. That's why bearings are often used: tight fit with low friction. But as @ADFinlayson has proved, freehand works as well. One trick is to draw the lines with a knife instead of a pencil, that might lead the saw to the right groove.

No matter which method you have the tools for, practicing is the key. That's the only way to make your hands steadier. And steady hands are handy in many other tasks in guitar building.