Grenadier1758 has some great advise here, his first recommended book is a good one for a starter, just be sure to read it.
Fatal mistakes;
* Don't be in a hurry to shoot it., It's a gun kit,, that can turn out really well,, not an assembly kit. I mean there's a little more to it than just screwing the parts together, sanding the wood an slappin stain and poly spray on it.
* Use a rasp, to take off the wood to profile, sand paper is for finish work, not shaping.
* Be careful around the lock panels with the sand paper, they have that shape for a reason
* Yes, The brass parts DO need to be filed for proper profile fitting. Use a heavy file for fitting, brass is easy to polish.
* file from the wood side into the brass, ya wanna take off the soft brass metal not the wood.
There's a bunch more, when ya get the kit, com`on back,, we'll help.
It's a good idea to read some of the "back page's" here, alot of questions have already been asked, with answers given by professionals.
The place is an "encyclopedia"!