Tuning seems easy, you just twist the knobs until the electronic tuner says you are in tune. Oh, if it were only that simple!
I recently had a workshop with Marla Fibish at the O'Flaherty Irish Music Retreat outside of Dallas, TX. Mandolins are notoriously hard to tune - you have to make sure both strings of the course are the same, there is a lot of stress on the instrument which moves the neck and changes the tuning, and the scale is very short which means that bridge placement is crucial. Marla showed us some of the tricks to tuning this finicky instrument that will translate to the banjo.
Banjos are not as hard as to tune, they have four strings after all, but they suffer from the same problems that a mandolin has plus they are banjos.
Why "plus they are banjos?" Among stringed instruments, banjos are a distinct minority in that they are an assemblage of parts instead of being an organic glued-together entity. The average banjo has between 70 and 90 parts all of which can come loose at some time. In addition they are all put together by external forces which have to be optimal for the the banjo to sound good.
Like other instruments they are also made of wood which is a material of widely varying properties and they are subject to environmental forces that can change the relationship of the parts.
The bottom line is that banjos can be just as squirrely as their owners and usually more often.
So where does this lead us when we want to tune?
The so-called Irish Tenor Banjo tuning is GDAE, lowest tone to highest, an octave below mandolin tuning and lower in pitch than the standard tenor banjo tuning of cgda. Since tenor banjo scales vary quite a bit from nearly 16" to about 23.5" (400mm to 600mm for those of you who are inch deprived) the string gauges vary quite a bit in order to reach these pitches. In addition action heights often vary for a number of reasons and the tension on the strings changes with a number of factors including string type and gauge.
If you add in the age of the instruments, the type of tuners, nut slot widths, bridge slot angles, etc., tuning suddenly becomes more complicated.
Other factors include your musical environment (i.e. how well the other instruments are tuned), the physical environment (temperature, humidity, changes in both) and you ability to hear and duplicate tones.
Actually it is a lot harder to tell you how to tune than it is to tune, so let's get started.
You need three things besides your banjo to tune well: an electronic tuner, a pick, and a good ear. In the old days before electronic tuners you had to have a really good ear if you wanted to be in standard tuning. The newer electronic tuners (the first one was a strobe tuner dating 1939 and they are still the most accurate) are cheap and accurate. I like the Snark series because they seem to react to banjos well but a lot of tuners work.
The pick is mostly important in that you are consistent in the way you strike the string and that you don't hit another string in the process. The harder part is developing your ear.
Being able to distinguish tones is a skill that every string player has to pick up at some time. Electronic tuners take some of the gross skills away - you don't have to know when you are close to A=440 anymore, the tuner will do it for you - but you do have to know when the tuning sounds "right." This latter skill is not a savant absolute pitch thing, it is more of an emotional response to when all four strings sound right. Banjos (or any other instrument) don't tune exactly to the mathematically correct notes that some tuners look for. For one thing nothing is a straight/parallel line with banjos so fret placement is not always reflective of absolute notes. Also, since banjos are resonating bodies they all have a natural tone that they resonate to. Some of this depends on the setup and especially the tension of the head so that certain notes will be louder or fuller than others. As a result if a string is slightly out of beat with the natural resonance of the banjo (which includes the other strings) then the banjo will not sound as good as you'd like.
Jazz (cgda) banjo players will often say that they like the third string to be slightly flat by a few cents to make sure that the banjo sounds best. Piano tuners tune the whole pianos close to perfect as possible and then the tune to the ear so the piano sounds the best. It humbles you when realize that every piano key has two or more strings.
Throw in testy tuners, slightly small nut slots, the wrong angle bridge break (the angle from the bridge to the tail piece), and the effect of string gauges then you have a less precise and more artful task when you tune.
The basic rules are: 1) Always tune from below the note, it lessens the chances of slippage later on; 2) tune the A string first as close to 440 as possible with the electronic tuner; 3) tune the other strings as close as you can with the electronic tuner and finish off with your ear; 4) banjos are ungrateful (expletive deleted) and go out of tune at the most inopportune times so be aware.
Here is a video showing you my method of tuning: