- Through learning Torah - Quoting from the Sifri, The AHS (Aruch Hashulchan) explains that learning Torah allows one to get to know Hashem, thus allowing one to develop feelings of love towards Him.
- Through appreciating Hashem's creations - The AHS quotes the Rambam (Hil. Yesodei HaTorah perek 2) who says that when one thinks about how amazing the creations of Hashem are, there is no way not to feel a sense of love and awe towards Him.
I just wanted to point out that by highlighting these two distinct ways to fulfill the mitzva of loving Hashem, we can perhaps gain a deeper understanding the Rambam in a few places.
The Rambam in Sefer Hamitzvot (#3) writes:
והמצוה השלישית היא שצונו לאהבו
יתעלה וזה שנתבונן ונשכיל מצותיו ופעולותיו עד שנשיגהו ונתענג בהשגתו תכלית התענוג
וזאת היא האהבה המחוייבת. ולשון סיפרי (פ' שמע) לפי שנאמר ואהבת את י"י אלהיך
איני יודע כיצד אוהב את המקום תלמוד לומר והיו הדברים האלה אשר אנכי מצוך היום על
לבבך שמתוך כך אתה מכיר את מי שאמר והיה העולם.
The Rambam here says that the mitzva is to ponder and understand Hashem's Mitzvot and His Actions - I would suggest that each of these words is a reference to one of the two points made by the AHS. "Pondering and understanding Haehem's Mitzvot" means learning Torah to understand Hashem's commandments."Pondering and understanding Hashem's Actions" means appreciating the complexity of Hashem's creations. These are the two ways to reach the level of loving Hashem (see also the language of the Sefer Hachinuch #418).
In the Mishna Torah, the Rambam also seems to make this distinction. In Hilchot Yesodei HaTorah (perek 2 which is quoted by the AHS) the Rambam explains that when one looks at nature and Hashem's creations, one cannot help but develop a love for Hashem.
Additionally, at the end of Hilchot Teshuva, the Rambam explains that when one learns Torah and understands the mitzvot, he can develop a love for Hashem.
We again clearly see these two fundamental ways to develop love for Hashem - by learning Torah, and also by appreciating the wonder of His creations.
There is more to say on the topic (like why the Rambam in Sefer Hamitzvot only quotes a source that has to do with learning, but apparently nothing about appreciating Hashem's creations)... but we'll leave it at this for now...
Shabbat Shalom