You can remember the meaning of 下手 with this easy mnemonic:
Mandy was all ready to have her ears pierced but her friend was slow to start the process. The friend carefully grabbed Mandy's earlobe from under (下) with one hand (手) and held the ear-piercing gun with the other.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.