ואמר רב יהודה אמר רב תלמיד חכם צריך שילמוד ג' דברים כתב שחיטה ומילה ורב חנניא בר שלמיא משמיה דרב אמר אף קשר של תפילין וברכת חתנים וציצית ואידך הני שכיחן
"And Rav Yehuda cited Rav: A Torah scholar needs to learn three things. Writing, ritual slaughter, and circumcision. And Rav Chanina bar Shelemyah in the name of Rav said: Also the knot of tefillin, the blessing of bridegrooms, and tzitzit.
And the other [=Rav Yehuda]? These are common."By כתב, Rashi explains it to mean the ability to sign one's name if he sits to be a judge or to give testimony. By שחיטה, Rashi explains that this does not mean just knowledge of the technical laws but actual physical ability to perform a shechita. And in terms of the knot of the tefillin, Rashi explain how there is a skill in getting it to look like a dalet so that all together the Divine name of Shakai is spelled out.
And what is meant by common? Rashi explains that since these are common, of course one would not how to do this automatically.
Another possibility is that all of these things are required to enable Jewish life, even out in the boondocks. And so someone needs to provide kosher meat, circumcise people, and so on. (Ketiva might then be a sort of safrut.) They would also need tzitzis, or someone who can fix tzitzis if it breaks. And so too the tying of the retzuos of tefillin. By saying that הני שכיחן, we are saying that even if the talmid chacham himself does not know one of these skills, someone else will be able to step in, and so it is not critical.
I admit that I don't possess all of these skills. I can sign my name. (Though I never took safrus.) I cannot perform milah. Though I studies hilchos shechita for semicha, I never actually slaughtered an animal. I can tie tzitzis, and know birchas chasanim. But I didn't really know how to tie the kesher shel tefillin.
This became relevant this morning. By accident, I left my tefillin bag in a place accessible to my 2 year old son, who took the tefillin out of the bag. My older son, Junior, saw this and rescued the tefillin. But then he noticed that the baby had really messed up the tefillin, by putting knots in the retzuos of the shel rosh. And so Junior "fixed" the tefillin for me by untying the knots. This morning, I discovered this, and Junior explained what had happened.
So, after borrowing someone else's tefillin for davening, I set about retying the knot. I have a double-daled knot -- that is, a box knot, and so I followed the instructions in this helpful video:
So, that is one off my list!