Completion vs satisfaction
完 emphasizes the endpoint of an action. When you 吃完了饭 (ate up the meal), the food is gone. When you 看完了书 (finished the book), you reached the last page. 好 emphasizes proper completion with a satisfactory outcome. 准备好了 (got ready properly) means the preparation went well; 关好门 means close the door firmly (not just shut it). The split maps onto "finished X" vs "X is done right".
我吃完饭了,现在准备好出门。
Wǒ chī wán fàn le, xiànzài zhǔnbèi hǎo chū mén.
I finished eating; now I am ready to go out.
Verbs that prefer 完
Verbs describing processes with a clear endpoint favor 完: 吃完 (eat up), 看完 (finish reading/watching), 说完 (finish speaking), 写完 (finish writing), 做完 (finish doing). These all have a natural finish line; 完 marks that the line was crossed. Adding 好 to these would change the meaning: 吃好 means "ate well" (enjoyable or sufficient meal), not "ate up completely".
Verbs that prefer 好
好 attaches to verbs where quality or proper completion matters: 准备好 (get ready), 关好 (close securely), 锁好 (lock properly), 坐好 (sit properly), 睡好 (sleep well), 穿好 (dress properly). For these, 完 would sound wrong or weak. 关完门 would mean "finish closing the door" as if closing was a long process; the natural phrase is 关好门 (close the door properly, all the way).
请把门关好,我要睡好一觉。
Qǐng bǎ mén guān hǎo, wǒ yào shuì hǎo yī jiào.
Please close the door firmly; I want to sleep soundly.
When both work with different meanings
Some verbs accept both complements with different nuances. 写完 means I finished writing (the document is done); 写好 means I wrote it well (the writing is good). 做完 is "finished doing" (task complete); 做好 is "did well" (quality outcome). In practice, 完 measures progress; 好 measures quality. For homework that you rushed through, 写完 fits; for homework that you polished, 写好 fits.
Negation and questions
Negating both uses 没: 我没吃完 (I did not finish eating), 我没准备好 (I am not ready). Asking if someone completed uses the affirmative-negative question form: 吃完了吗 (did you finish eating?), 准备好了吗 (are you ready?). These two questions are among the most common in daily Chinese; learning them as a pair locks in the 完 vs 好 distinction.