“You shall not add on to it” (Devarim 13,1)
It is a negative commandment, that we may not add on to the written Torah and not to the oral Torah and about this it says “You shall not add onto it”. How can one add on? An example is someone who lays two pairs of kosher tefillin on his head and on his arm, or one who makes five sections in the tefillin (instead of four), or one who takes two kosher lulavim in his hand or one who sits in a Sukkah after the festival with the intention of performing the commandment of Sukkah, even though he knows that the time of the commandment has passed.
Amongst the roots of the commandment, is that the Master who has commanded us about the Torah, may He be blessed, is infinitely complete and all His deeds and all His commandments are complete and good, and adding to them is taking away, and how much more so subtracting from them is taking away. This concept is extremely clear.
This prohibition applies at all times, in all places, to men and to women. One who adds on to the commandments, transgresses this negative commandment and if he does so in front of witnesses and with a warning he is penalised with lashes (malkus).