Hey There!
Okay, first off always remember something basic. [Topic First, Comment Second] When signing, you always say what you are talking about first and then what you want to say about it. So if you wanted to say, "The cat is brown." you would sign "Cat (index to cat) brown."
Now more specifically, there are multiple ways you can sign something. It's just like English where you can say the same thing but in different word orders. I believe the most common form is Subject-Object-Verb. Again, this can be doen in many different ways as long as the main Subject or Object that you are talking about is first and
then what you want to say about it. There are certain things like when talking in a formal sense or when just chatting with a friend but I really don't know all the rules to that. Hopefully someone else could shed some light on all that. Though, I rarely notice anyone changing the way they speak when it's in a formal setting so maybe it's just mainly used for interpreters. (Correct me if I'm wrong, please.)
As far as repeating a sign to make it a verb. I have heard of this many times and I myself do this. I have run into a lot of Deaf people who have never heard of this though. So I'm not sure if this is something fairly new or it's just not common. But yes, it is. If you sign chair once it means chair but if you sign it twice it means sit. It also works (from what I've seen and do) if you want to sign study, to sign it once it means study but to sign it twice means studying so it makes the sign into present tense. (I hope that makes sense.)
Good luck! And again, if I'm wrong PLEASE correct me! I'm not great at the grammar side of things. I just sign without thinking much so I never really thought about it.