how to use functions in my theme

Hi, I'm traying to create a custom template, but i can't understand how to use the functions in template-function.php

So, what i'm doing by now its copy and paste the function that i need to my function.php file, then, rename that function and make the change that i need.

Is this ok?
is there any manual or tutorial?

I read the Themeing_MarketPress.txt file so please dont tell me i have to read this file.
Also i download a marketpress template, but i don't know if i'm doing the right way.

Tks in advance