How can Karaites know how to make Tefillin if the written Torah doesn tell us how?
We can not know how to make Tefillin from the Torah because Tefillin were only invented by the Rabbis 1500 years after the Torah was given. The Torah commands us to treasure the commandments as jewels, not to strap leather amulets onto our heads and arms. For more information see: http://www.karaites.org.uk/phylacteries.