Before you run off to Home Depot for some parchment, ink, and goose quills, you might want to consider a few pesky details about mezuzot.
Firstly, as you can see reflected in the Kitzur’s wording here, many authorities hold that it is a positive commandment to “affix” a mezuzah and make no mention of writing it. Some authorities do assert that the writing of a mezuzah is part of the mitzvah. From that perspective, one could apply the principle of: “It is a greater mitzvah to perform a mitzvah by oneself than to appoint an agent to do it for him.” However, even these authorities agree that the main mitzvah is fulfilled later, at the moment of placement, or by the fact that the mezuzah is there. So, even according to these authorities, by your involvement in affixing the mezuzah by yourself, you still have a chunk of the “greater mitzvah.”[1]
Don’t underestimate the complexity of the writing process. The hardest part about learning to be a sofer (scribe) is not the mastery of its specific calligraphy, but rather becoming expert in the many laws that apply to mezuzot, and all the other texts that are written on parchment.
Although one does not have to be a rabbi to practice as a sofer, a person must undergo rigorous training and, in modern times, is tested and apprenticed in order to be certified by regulatory bodies. A sofer must be a G‑d-fearing and pious person. A mezuzah, and especially its Divine names, must be written with utmost purity and intention. It is therefore customary that the sofer immerse himself in a mikveh (ritual pool) before beginning his work.
Moreover, the Talmud accentuates the great responsibility that rests on the shoulders of the sofer entrusted with this spiritually powerful mitzvah:
Rabbi Meir [reported] …When I learned with Rabbi Yishmael, he said to me, “My son, what is your profession?” I said, “I am a scribe.” He said to me, “My son, be careful, as your work is the work of Heaven, for if you leave out one letter or add one letter, it is as though you have destroyed the whole world.”[2]
Calligraphy is also a consideration as the Torah enjoins us to beautify the mitzvot we perform. Even if you are a talented amateur, it is unlikely that your writing will be as pleasing as that of a trained professional, and some authorities opine that affixing a beautifully written mezuzah may take precedence over the value of your personal involvement in the writing.[3]
You may have seen that at a celebration marking the completion of a Torah scroll, several letters are left over to be filled in by those present as this may give them a part in the mitzvah of writing the Torah. But this custom is not found regarding the writing of a mezuzah, possibly because of the risk of a layman making an error that will be difficult to fix.[4]
[1] Kidushin 41a. Even according to those (Mishneh Torah, Sefer Torah 5:7; Ritva, Shabbat 131a) who hold that the writing is only a preliminary step to the actual mitzvah of placement, it may still be a “greater mitzvah” to be involved personally from the writing stage. See Ha’ameik Sh’eilah 169.
[2] Sotah 20a.
[3] Chayei Adam 68:7.
[4] See Agur B’Ohalecha 2:8.