Take pictures of the instrument - ideally from all sides, with a rule next to it.
For unfamiliar instruments, a close-up shot of the keys may show which are struck
from above or underneath.
Muchatera Mujuru's mbira from the Tracey Instrument Collection (TIC #125).
Take notes about the instrument's maker, its build date and current location, and its owner (including contact data).
How is tuning called (or how does the owner want it to be called, if there's no established term)?
Try to find a quiet spot without ambient noise, ideally a 'dry' room with little reflexion, and high absorption and diffusion.
At home, I sometimes build a little 'cave' out of chairs, blankets and cushions,
just big enough to comfortably fit instrument and recorder.
In the field, recording inside a car might be an option.
Damped/mute the buzzers.
Even though it is an important component of the sound,
in the synthesized playback the buzzing quickly adds up in unnatural ways.
Cloth and rubber bands usually work well.
On Matepes with buzzers inside the bell resonator, I push in a a piece of cloth with a ruler,
wrap around the buzzer rod, and pull it out again.
Occasionally, if I find the buzzing or its construction remarkable,
I sample the instrument twice, with and without buzz.
Check the recorder settings.
Record in a uncompressed (or lossless) format, with the best possible audio quality (44.1kHz, 16Bit stereo, or better)
Identifying Instrument and Keys
Speak out loud all information needed to unmistakably identify the instrument, for anyone listening to the recording later.
Quickly record all keys from left to right, as they are mounted on the soundbard, speaking out their number aloud.
This 'index', together with the picture, may later unmistakably identify keys.
Record 3-4 good takes for each key.
Let them fade out until the instrument is completely quiet
(sometimes there are very long resonances within the soundboard).
Pay attention to ambient noises.
Repeat as often as needed, better too many takes than too few.
The reason for recording multiple takes of each key is to enable the SR playback engine to randomly pick
a different take each time a virtual key is struck.
This makes the playback much more lively.
Keep an eye on the recording level.
The volume should be even, but sufficiently high while avoiding clipping.
Speak comments as needed, e.g. when starting with a new rank of keys, repeating takes, starting over, etc.
It will later save a lot of time when editing the recordings.
If you're familiar with the instrument layout, sample keys consecutively in the different ranks,
so you move your hand very little.
This can help to keep volume and attack sound more even
If keys are clinking, try to pluck them a bit softer until the sound is clear.
Later in the audio software the volume can be raised again.
Loose keys - if you cannot fix them nor play them softer -
sometimes can be made ring again by pushing them against the metal bar (or slightly above the point where
they cross the bar), with the finger or a pointed object.
But beware not to accidentally affect the pitch of the key.
None of the above is important enough to not capture an instrument's tuning if there's a unique occasion.
Even a mobile phone recording in a noise environment can still be sufficient to synthesize decent playback
and analyze the instrument's tuning.