Under the slot-based conversion on pg. 26, cantrips aren't a thing. If that's breaking for your game, dispense with the "spells’ levels are equal to the number of words in the spell" rule, and use the table on pg. 15 as a guide for leveling your spells.
The Detriment Rule (pg. 42) can get you more resonance out of a spell outside of the harmonics; there are ways to optimize your harmonics up to the levels described by the chart on pg. 15, but the Detriment Rule is an easy way to do it without sweating too much over them.