Tuning a glass harp is very easy, most people think the opposite because this instrument seems very intriguing therefore everything related to it must be difficult.
All one has to do is adjust the water level of each glass consulting a chromatic instrument tuner. If the pitch is high add water, if it's low take some out. It can be done by anyone even if they are not musicians or don't have a particularly sensitive musical ear.
Anyway there are some refinements that can improve the musicality. I think an extended tuning sounds much better than the regular one. I tune the first octave to 440, second 441, third 442. This gives more brightness to the melody while playing an accompaniment on the low notes. Also some glasses sound different to others, a bit like having a guitar with a nylon string, an acoustic one, an electric one, and so on. You can tune it perfectly but when you play a chord it won't sound clean. But if you give a bit more tension to the ones that sound less shiny (nylon) it will sound better. So if a glass sounds a bit dark (cavernous) even if the tuner says its on pitch, trust your ears first.