When you kindle the lamps (Numbers 8:2).
The word beha'aloscha here means "when you kindle." But this word, from the root aloh, also means raising, or lifting. Rashi uses this derivation to add two meanings. One is that the Kohen must hold the fire to the wick until the newly kindled flame is strong enough to rise up by itself. The other is that there were steps in front of the Menorah upon which the Kohen would stand when cleaning and filling the lamps.
In truth, the two explanations are closely related. The Menorah represents the enlightenment which comes from the study of Torah. Aharon and his sons are particularly delegated to learn and teach the Torah, as the verse states: They will teach Your laws to Yaakov and Your Torah to Israel (Devarim 33:10). When they teach, they must see to it that their teaching should "rise of itself" in their students - that is, that their understanding must grow to the point that they understand not only what they were told, but clearly understand all its applications and ramifications. We see this from the word beha'aloscha: Any mitzvah to light the Menorah would obviously demand that we see to it that the lamp remains lit, and that we may not stop lighting until we are sure it will stay lit. Beha'aloscha tells us more - that we cannot rely on our evaluations, and that we may assume nothing. We must know with absolute certainty that the flame is strong and increasing in strength. Exactly so is our responsibility when making halachic decisions and teaching and influencing others. We must keep our hand in the matter until it is absolutely clear that they understand and are growing in Torah and good deeds.
The same may be learned from the cleansing procedure. The cleaning could have been done without recourse to a set of steps, because the Menorah was only eighteen tefachim (approximately five and one-quarter feet) high. If we could rely on presumptions and evaluations, a cleaning done from ground level would allow us to confidently say the Menorah is perfectly clean and ready. But we may not rely on anything, and we must know absolutely, by looking downwards into the lamps, that they perfectly clean and ready. Similarly, when we teach others to avoid sin and develop good character traits, we must do so fastidiously and with perfectionism.
Rashi explains the verse Aharon did so (8:3) to mean that Aharon did not alter from his instruction. This means that although we know that if Aharon was confident this would mean complete certainty, he did not rely on his confidence or his evaluations, and waited until he actually saw that the lamps were clean and that the flame grew. This is, indeed, a novel and though-provoking explanation, because Aharon's certainty based on his confidence in his evaluation may be no less reliable than visual proof - and even so, he waited for visual proof.