If I understood correctly, both 完 and 了 can be used to indicate a completed action, so what is the difference in use? How do we properly use 完 in a sentence? Which contexts would be valid?
You can use 完 and 了 together or separately.
完了 can have 3 different meanings/uses.
没完 can be used to describe things left undone, or something never-ending/eternal. E.g., 活儿还没完 (Work is not done/finished yet) or "說話沒完的人" (a motormouth, someone who talks to no end).
没 and 完 can be used together to indicate an action that has yet to be completed in this pattern 没 + V + 完 for example:
To mean I haven't finished watching/reading it or I haven't finished eating (it)
You can also combine the two in this case it would be:
To mean the same as above.
However the following:
This means I haven't seen/read (it) or I haven't eaten (it)
I (still) haven't read this book or He/she hasn't eaten their ice-cream.
I would translate 完 as "complete," and 了 as "done." 完了 would then be a very emphatic "completely done."
The other answers give some good examples, so I'll try to provide an answer from a grammatical perspective.
完 can be used as a resultative complement meaning "to finish doing something". You put it after a verb to mean that you finished doing that verb. 「吃完」 means "finish eating".
了 has many different uses, but the one that I first learned was as a past tense marker. 「吃了」 means "ate".
Now, to talk about something that you finished doing in the past, you combine the two. 「我吃完了」 means, "I finished eating."
There's a ton more detail to learn with these two words. I suggest you look at the Chinese grammar wiki complements page, the completion with 了 page, and maybe the many uses of 了 page. I expect most beginner or early intermediate textbooks will cover these topics as well.
All answers miss an important point: the two words are total two different stuff at all! 完 is a verb while 了 is an auxiliary word.
完 in case of this question is used as a secondary part of a compound verb, it denotes the result of the dominated verb. Such kind of compound verbs are called Resultative Compound Verbs. In most situations, say 写完了报告, 了 is used with the whole compound verb instead of just 完. They are not connected as they seem!