Answer: That’s a good question! Actually, the main mitzvah-act of donning the ‘shel yad’ is placing the ‘bayis’ (box) around the biceps; the requirement to wind the ‘retzua’ around the arm seven times, as most of us do, is not absolutely crucial to the fulfillment of the mitzvah. Therefore it is reasonable to argue that one should don the ‘shel rosh’ before winding the ‘retzua’ around the arm altogether. (Note: In fact, there are some primary sources which advocate that.) The practical halacha is that we may wind the seven winds around the forearm but stop before winding around the fingers and hand. The common practice is to simply wrap the excess ‘retzua’ around the hand in a temporary manner, then don the ‘shel rosh’, and then complete the winds around the hand.
One should not even remove the ‘shel rosh’ from the bag until he has donned the ‘shel yad’!
The Ashkenazic custom is to stand throughout the entire process of donning the tefillin, and their removal.
Question: If one intentionally removed his tefillin for a short while, or if the tefillin moved involuntarily from their proper place on the arm or head (i.e. slipped), are new brachos required when they are replaced?
Answer: We dealt with the identical questions concerning the talis back in the halachos of tzitzis, and the answers for tefillin are partly the same and partly different: If the tefillin accidentally get moved from their place, new brachos are not required when they are replaced. The same is true if one took the tefillin off for a brief while during davening, with the intent of putting them back on. However, if the person took them off because he needed to use the restroom, then new brachos are required. This is the point of departure between the talis and the tefillin. A talis can technically be worn in the restroom. (Ed: we wear our tzitzis there, don’t we?) Tefillin cannot. Therefore, with a talis (even though the custom is to remove it) the use of the restroom does not constitute an interruption; with tefillin, it does! (Ed: One may not be permitted to make the brachos on the tefillin just then, depending on where he is in the middle of the davening.)