You can use a banjo capo to change the key that you are playing the banjo in by simply clamping the capo onto the neck of the banjo. One disadvantage you will encounter when using a banjo capo is that the fifth string, or the top string on a five-string banjo, is not as long as the bottom four. This results in a separate capo being required for the fifth string. You can eliminate the need for this separate capo by tuning the fifth string to the desired pitch once the capo is placed in the desired position on the banjo neck. Once the banjo capo is placed on the neck of the banjo, the instrument is played as normal.
A banjo capo is used in the same manner as you would use a capo for a guitar, with one exception: the capo designed for use on a banjo commonly involves the use of two parts. In order for you to properly capo the fifth string, a separate fifth-string capo is commonly used in conjunction with the common banjo capo. The fifth-string capo is installed by screwing a bracket onto the neck of the banjo. Once installed, you simply slide the fifth-string capo down the neck as many frets as you intend on placing the banjo capo and tighten it in place on the fifth string.
While many capos are self-adjusting, some are not and require proper adjustment to produce a quality sound as well as to avoid damaging the banjo neck. With a self-adjusting capo, you simply squeeze the capo open and place it on the neck at the desired fret that you want to play at. With an adjustable, banjo capo, you will need to position it on the banjo's neck at the desired location and adjust the tightening nut until the capo becomes just snug while in the open position. Once properly adjusted, the banjo capo is clamped tight on the neck and you can play the banjo exactly as you would without the capo.
If you do not wish to drill the neck of your banjo and install a bracket on it for the fifth-string capo, you can simply install the main capo onto the neck of your banjo and tune the fifth string to the proper pitch to work with the a capo in place. This can be done using an electronic tuner, or in the more traditional method of simply tuning to ear. For this method, you place the banjo capo on the instrument and simply pick the fourth and fifth strings until the proper intonation is received from the pair.