I've been told that stuff is not countable since that I cannot say one stuff. But which is / are the proper forms for using the "stuff" word?
"Stuff" is not countable. You can say "some stuff" or "lots of stuff" or "a bunch of stuff". But you can't say "one stuff" or "two stuffs". Just like you can't say "one dirt" or "two dirts".
"Stuff" is a very informal and vague word. It is generally used to refer to vague, undefined things. Like, "There was a lot of stuff in the room". Or, "What did you do in school today, son?" "Oh, we did a bunch of stuff."
What are you trying to say by "one stuff"? If you are able to count something, you can probably be more concrete. Like, "There was one box of paper on the shelf" as opposed to WRONG "There was one stuff on the shelf".
If the objects you are referring to are unknown but countable, you can use a general word like "things" or "items". Like, "There was one thing on the shelf."
Note that "stuff" should not generally be used in formal writing. For example, you would not write, "We diluted the solution to 100 grams per milliliter, then added nitrogen sulfate. Some stuff fell out." It would be much more common to say, "A white precipitate was produced." or something of that sort. (That said, occasionally technical writers will use the word "stuff" in a whimsical way. I recall reading an astronomy book that referred to the chemicals making up stars as "star stuff".)