What is involved with tuning the pipes?
Because of the continuous background of the drones, any deviation from "just" temperament can be immediately obvious.
There are a couple aspects of tuning that should be discussed.
First, in order to tune with any accuracy, the pipes the piper must be able to make his pipes create a steady tone. This requires skill on the part of the piper in blowing and a well set-up bagpipe. While tuning, the piper must be able to blow his bagpipe at the same pressure that is used while playing while doing each step in the tuning process. This sounds easy, but is often a problem. Many less experienced players will blow at a low pressure while tuning, a medium pressure while playing with the band, and a much higher pressure if you ask them to play as soloists - or in the competition circle with judges around them!
Secondly, there is no standard of pitch and manufacturer sets their own standard depending upon their taste. The low A on the chanter will typically be slightly above concert B-flat. To make matters more confusing, the absolute pitch of the bagpipe is affected by temperature and humidity (see Why/how do my drones change pitch with temperature?). Consequently, it is very difficult to "predict" what pitch the instrument will produce in a given environment.
Having recognized these problems - the player must first get low A and high A to be an octave apart in the environment in which the instrument is to be played. This is called "balancing" the chanter. (Note: In practice high A is often left a little flat on beginner and intermediate players to compensate for over-excited blowing during a performance.) This is done by adjusting one drone, usually the outside tenor, to the low A of the chanter until there is no more of a "beat" or "wah-wah" in the sound intensity. (Note: Low A is often used as it's easier to hear the beats, but this can be done the other way around if the piper has a very good ear.) Then high A is checked for consistency against that drone. If partial covering of the high A hole with the thumb, improves the match, the high A is too sharp and the reed must be lifted in the seat. If the sound gets worse, then high A is flat and the reed must be pushed into the seat. Either "lifting" or "pushing" affects all the notes on the chanter, but affects the high notes to a larger degree than the low notes. Consequently, the whole process (including matching the drone to the altered Low A) must be started over. When high A is acceptably close to low A, the chanter is balanced.
The other two drones are then matched to the first by elimination of the "beating". At this point, it is possible to "flatten" individual notes by application of tape to the top of the highest open tone hole on the chanter.
Problems with tuning are generally related to ability of the piper although some sets are easier to tune than others.
There is also a functional limitation of the human hearing that beat patterns of less than one beat in five seconds cannot be well judged.
The tape you use should be flexible in all weather. You should be able to move it and not leave a "sticky" residue. As to what kind of tape to use: