1. Explain it simply
A result complement hugs the verb: verb + complement is one unit, like 看完 or 听懂.
For a completed action, put 了 after the whole unit: 看完了, 找到了 (never 看了完).
To say it did NOT happen, use 没 before the verb and drop 了: 没看完, 没找到 (never 不 for a realized result).
2. Show examples
我听懂了老师的话。
Wǒ tīngdǒng le lǎoshī de huà.
I understood what the teacher said.
懂 = the result of listening
你做完作业了吗？
Nǐ zuòwán zuòyè le ma?
Did you finish the homework?
了 follows the whole unit
我没找到我的书。
Wǒ méi zhǎodào wǒ de shū.
I didn't find my book.
没 before the verb, no 了
3. Share a tip
Keep the verb and its complement glued together as one block. Add 了 after that block for a finished action (看完了), and negate with 没 in front of the verb while dropping 了 (没看完) - never split them with 了 or 没.