Question: Am I allowed to make a sign on my computer on chol hamoed to advertise a shiur?
Answer: The Mishna (Moed Katan 11a) teaches us that ma’aseh uman (the work of a craftsman) must not be performed on behalf of an individual on chol hamoed. Only ma’aseh hedyot (the work of an ordinary person) that is necessary for that chag is allowed.
Rambam (Yom Tov 7:14) writes that personal letters are considered ma’aseh hedyot and so may be written, though the Rema (OC 545:1; 5) adds that one should do so in an unusual manner. Thus, the Magen Avraham (545:21) suggests writing the first line on a slant, while the Mishna Berura (Shaar Hatziun 545:8) writes that one can write in a different script. He emphasises (Biur Halacha 545:1), however, that one should only write when absolutely necessary.
R’ Moshe Sternbuch (Moadim Uzmanim 4:301) considers printing to be ma'aseh uman and therefore assur.
Nonetheless, the Eshel Avraham (545:7) writes that while using stamps and seals on Shabbos would be an issur of kosev, writing, one may use them on chol hamoed if necessary, as they are considered to be ma’aseh hedyot. R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 66:55; n209) extends this to using typewriters, etc., writing that R’ Shlomo Zalman Auerbach compared them to stamps which are considered to be ma’aseh hedyot. Likewise, R’ Ovadia Yosef (Yabia Omer 8:48) writes that one may use a computer for mitzva related tasks such as typing Divrei Torah (See Baer Moshe 7:39).
In conclusion, one may use a computer to design a sign for things related to the chag.