Essentially this is a piece of wood with an old drum head and some strings attached, it was under 30$ for me, and though it is not a finely crafted piece of art it does pay homage the original "banjos" which were made by slaves in early america with what was available (of course based several african instruments).
Step 1: Gathering Materials
You probably don't want to use an actual 2X4 (unless you're going the cut it vertically) Mine was 1 3/8" by 1 5/8", if you chose to buy or cut a piece of wood I recommended using a standard guitar width based on nut width (e.g. 1 1/2" for a classical guitar). The length of the wood is also important, to calculate your length you need to factor in your scale length (distance between nut and bridge, I used 25" which a fairly common for an acoustic guitar, "fender scale length is 25.5, "Gibson" is 24.75", and lots more- use what ever), your headstock length (the headstock is the end where the tuning pegs are, mine was to short at 4", so cut it long!), half the diameter of your drum head (you pace the bridge in the middle, meaning only half the head is encompassed in the scale length) and a tail (where your strings attach, mine was 3" and worked well). eg L=Sl+Hs+(Dh/2)+Tl using a scale length of 25" a headstock of 6" a drumhead with a diameter of 14" and a tail of 3" is L=25+6+(14/2)+3 giving a total length of 41".
Other than that you will need a drumhead, I had a fairly "Traditional' (I have know idea what word) drumhead that could be mounted easily, however any drumhead will work, you just may need to cut slots for it or cut the head it self.
For bridge, strings, nut and tuners I just bought the cheapest available, all the parts were guitar, except the banjo bridge. The stings are nylon so i don't break my instrument.
You also need a variety of screws, bolts, nuts, glues and other things that you may need to hold it together.
I used only hand tools for this project, nothing was specific to instrument making, and you may want to use different things depenting on the situation
You will need
At least one saw (preferably fine for small cuts)
chisels/wood carving tools (smaller is better!)
Lots of different grades of files
Tape mesure and ruler/square
A drill (and bits big enough for your tuners
Step 2: Primary Cuts and Mesuring
The best thing to start with is fitting the parts together (at least mounting the drumhead and defining your cuts).
You will want to cut out a cavity in your wood for the drumhead to resonate. The measure for this cavity you will begin by placing the end of your drumhead at the "end" of your tailpiece (see photos) you will the mark each side of the head (rounded profile), I then moved and inch inwards (away from the tailpiece on the end pointed towards it and away from the headstock on the headstock side), you can use whatever length distance you want- I strongly suggest recessing the grove where the rim of the drumhead sits, so you don't get stupid high action or strings that hit the edge of your drumhead. After that you can mount the drumhead.
Next you will want to mark for all your profiling/mounting/cutting. To start you can get a rough idea of your neck profile based on where your drumhead is and the nut marking (end of headstock), I shaped my neck into a V, for lack of ability/desire to cut all the wood, so I measured each side and left a narrow ridge in the middle. After marking the back of the neck you'll want to mark the depth, this is how far down you will cut ( I cut a straight diagonal line between my marks and the round the flat parts.
You can then cut for the nut, I chiseled out a small cavity for it to fit into the inward side is the "end" of the headstock. Note that this to be done based on measurements of the nut and should not be performed without it
Lastly I marked string spacing , this not is necessary if you are using a standard guitar dimension/nut (I had to cut my nut to new spacing).
Step 3: Shape Your Neck Profile
I order to make the banjo a little more playable I decided to give my neck a shape similar to what is used on real stringed instruments (well kinda). To begin you will need a to cut (or file) into your neck within your markings, straight in (on a diagonal). After leaving some sort of cavity you can place a saw in going straight up and down (length wise) your instrument and cut. You will likely need to remove the saw and redo this several, remember try to go maintain a straight line along your marking, the final shape will be achieved by filing. You should note to only do the middle section and the ends of your neck, those come at the end.
After you achieved a rough but cut neck, you can begin filing, I used a very rough file and made the surface as flat as possible. Once you have a filed both sides of the neck to level you can end it. I used a variety of small chisels and wood working tools the create a nice smooth transition into the headstock/end.
After you have completed removing wood and the neck is near it's final profile you'll want to file curves and sand it using fine sand paper ( I used grades going up to 360 grit). This will create a smooth, comfortable neck.
Step 4: Modifying the Nut and Bridge
If you are using standard neck width and nut spacing you can skip the first half of this step, however if you are like and have an odd size neck you will neck to cut and respace your nut. You will need to modify the spacing of your bridge to match your nut for this project, as the neck shape does not change.
I used a classical guitar nut measuring 1 1/2", my neck on the other hand was 1 3/8". To begin simply cut the end off the nut so it matches the width of your neck (I'm respacing the strings anyways, so cutting the nut doesn't matter). The string spacing marked on the headstock now comes to use, as you can place your nut center with the neck and transfer your marks (a clamp is a good idea). If you really want you can measure the spacing. You can see from the photos that I cut my grooves on a different side of the nut, that was to lower the action and prevent the strings from falling into the wrong groove.
The bridge can not be cut, it must remain full length, I used a banjo bridge, so it was necessary to modify it to fit six strings. To begin you must first measure evenly on both sides from the edge of the bridge to the edge of the neck (mark the width of the neck on the bridge). Once you have identified your center you can measure from the headstock markings, the nut or measure based on either. After that simply saw your grooves.
Step 5: Installing Tuners
To install the tuners you will likely have to cut out a chunk of wood to reduce the thickness, you can check the manufactures website or the packaging to figure out how thick your headstock needs to be. Once you have figured that out mark on your headstock how far down you need to cut to achieve that thickness, you can cut out a section from either the "fingerboard" (playing side) or the profiled section, I recommend cut out from the fingerboard side, that's not what I did but it probably works better. Once all of your marking is complete you can then cut, you should have a boxy flat section now. File it all thoroughly.
You should check the fit (length wise) of you tuning pegs before drilling, if it looks good you can continue.
Measurements for you tuners should again be located on the packaging, if not measure the diameter and distance center to center of tuners (If you have one piece sets of three like me). After taking down these measurements mark you headstock (horizontal lines) and chose your drill bit, the horizontal placement of the tuners is at your discretion, once mark you will have cross points to drill into. Check it all.
Once you tuners fit perfectly (or at least fit) into your headstock it is just a matter of attaching them, screws come in the box, figure it out.
Step 6: Stringing
The banjo is now near completion, all that's left is attaching the strings and making sure she holds up.
The strings that I used (and recommend) are basic nylon guitar strings with no end. To attach them to my instrument I simply used screws and tied a not at the end of each, I tightened the knot and screwed the string down.
Placing the bridge close to the desired location (end of scale length) run the strings up to the headstock, run them over the nut and attach as one normally attaches guitar strings. Tighten them until there is moderate pressure on the bridge, them run your tape measure down from the inward edge of your out to the inward edge of your bridge to your scale length (e.g. 25"). You may also need to add some stuff to ensure proper alinement of strings
Step 7: Conclusion
Once you have tuned it up and everything works you're done. This certainly isn' much of a Super Earl however it's cheap, easy and gets the job done. If I ever make another I would do a few things differently, i would recess my drumhead level with my neck, make the headstock longer, the entire thing wider and maybe even lacquer it. Anyway's that's the end, change what needs and have fun. Critism is always welcome