I suspect many have already heard me say most of this, but it's worth repeating....
Building folders doesn't only demand that you learn new methods and techniques, but, in order to be successful, requires a change in how you think. To explain, when building straight knives, most people think in terms of fractions of an inch. The need for precision with folders forces the maker to teach themselves to think in fractions of a thousandth. Which isn't a bad thing.....because it will maker your straight knives that much better.
Steve gives sound advice on books and other sources. PLEASE do not depend on YouTube! I have watch video after video on Youtube that claim to "how to make folders", or "getting started making folders". They are so full of misinformation that I have to wonder if the individuals putting them out there can even spell "folder". The alternative? Seek out information/texts from established folder makers! It will be well worth the effort.
OK, now, any new folder that a maker produces will require a pattern, and likely a few prototypes. I personally like making folder patterns out of thin Plexiglas, or other transparent materials....that allow you see the travel of the blade within the handle. This will allow you to see where blades, stop pins, locks, etc set in both open and closed positions, and allow you make adjustments more easily. Once you get a pattern you feel is what you want, transfer it to steel....that way you don't wallow out holes when using it for a pattern.
Let's talk tools!
First, get yourself a GOOD set of digital calipers, micrometer, or both! You will literally wear them out if you build enough folders! Next,
a wall chart with Inch/Metric, tap drill sizes & Decimal Equivalents...... You will find you use it EVERY time you build a knife or folder.
Here's likely the most important tool.....
a GOOD drill press, or better yet, a benchtop mill.
It's about accuracy of holes..... that means not just "round" instead of "oval" shaped holes (this has more to do with the quality of drill bits then anything), but also hole being drilled "square"....or at true 90 degrees. Most drill presses these days have "flex" in the tables....which mean that holes drilled on them are usually NOT square. Do this to a folder frame or blade, and it will drive you nuts trying to find what went wrong. The solution for most is a benchtop mill, or a very high dollar drill press....the kind that is massive, weighs a lot, and is solid.
If you're going to get serious about folders,
you WILL eventually want a surface grinder. I know, I know.....EXPENSIVE. Yes, but you can literally try to hand sand for DAYS on a granite surface plate, and still never get a folder blade anywhere near as flat as a surface grinder will in a few moments. What's "flat"..... let's just say this.... in my experience, if a folder blade is as much a ONE THOUSANDTH out of flat when finished.....the folder simply won't work....at least not in an acceptable manner.
Those are some of the major pitfalls to avoid, and please don't let it deter you.
You do not have to have all of that to start, I'm just telling you that without them, building folders is "challenging" to say the least. Can you do it? Sure, I did. But I spent literally WEEKS more time and effort, for folders that were only "so-so". It's just one more challenge in the never ending learning curve of Knifemaking..... and on the bright side....it give you and excuse buy more tools!!!