In some cases mnemonics work, in others, simply knowing the components works. Sometimes both concepts combine, such as in 木 (tree), 林(woods) and 森(forest). Sometimes it doesn’t work at all, often with kanji compounds, and you just have to memorize it and the various cases for different readings.
Knowing that 悪 (evil, bad, hate) is a weight or i-beam crushing a 心 (heart) only takes you so far. It won’t tell you if it’s aku, o, or waru. You’ve gotta learn it and use it.