The Pitum HaKetoret lists the eleven ingredients that made up the incense that was used in the Bet HaMikdash, and states that if the Kohen who concocted it missed even one ingredient, he was liable to death. Mahari Abuhav explains that in the absence of the Bet HaMikdash, reciting the Pitum HaKetoret comes in place of the actual incense-offering. As such, the Bet Yosef (§ 133), quoting Mahari Abuhav, states that it should only be recited when read from a written source, and not by heart, so as not to accidentally omit any of the spices. The Rama (Darke Moshe, 133:4) writes that it is for this reason that the liturgy of many Ashkenazic communities does not include the Pitum HaKetoret at all. Nevertheless, the Bet Yosef says that the death penalty is only imposed on a Kohen who offers an incomplete incense and not apply to someone who is merely reading the list of spices. As such, one may recite Pitum HaKetoret by heart, but one should still do so with proper concentration.
Interestingly, the HIDA (More BeEtzba, § 73) says that it is proper to enumerate the ingredients with one’s fingers as they are being read, so as to ensure that none is omitted. Furthermore, the Ben Ish Hai (Parashat Miketz, § 8) writes that since the Kohen offered the incense with his right hand and that the right hand represents kindness, one should enumerate the spices with the fingers of one’s right hand only. Alternatively, Rabbi Haim Palag (Kaf HaHaim) says that all of one’s fingers should be used so that both hands take part in Mitzvah of the Ketoret.
Summary: The Pitum HaKetoret may be recited by memory, but it is proper to use one’s fingers to enumerate the components of the Ketoret as they are being recited.