Mishnah Berurah — משנה ברורה

Both standing - See in Magen Avraham who attempts to make a compromise [by saying] that putting on the hand tefillin should be done while seated, but the blessing should recited [first] while standing. But Eliyah Rabba in the name of Rasha"l in Responsa, siman 98, wrote that who is there greater than R. Shimshon of Kinon, who after learning all of Kabbalah would still pray like a young child, for one who cannot understand its' secrets properly will come to "uproot saplings" - therefore, the blessing and putting on of the tefillin should both be done standing. Biur HaGra demonstrates that even according to the Zohar it is permitted to put the hand tefillin on while standing. Therefore, one should not deviate from the [accepted] practice. Knesset HaGedolim wrote in K'lalei Haposkim that anywhere that the Kabbalah and Zohar are in disagreement with the Gemara and poskim, one should follow the Gemara and poskim. However, if the [opinion according to the] Kabbalah is more stringent, then one should be also be stringent. If [a halacha] is not mentioned in the Gemara and poskim, even though it's mentioned in Kabbalah, we can't force people to adopt that practice. A halacha which does not have an opposing view mentioned in the Gemara and poskim, one should follow the Kabbalah. Also, when there is an argument between the poskim, the [opinion mentioned in] Kabbalah should be the decisor.