Hi, is this one of the AHM plastic kits from the 1960's (There were at least two, 1) Casey Jones 4-6-0 and 2) an 0-8-0 IHB switcher)? Or are you talking something a bit older and all metal? You will likely get two different responses. The AHM kits would be pretty much like any modern plastic model kit I would think. Just remove the parts from the sprue , clean the flash off, maybe a light fitup and follow the instructions.
On the other hand, older kits from the 30's - 60's that would have been all metal, either brass, bronze, tin, cast zamac or otherwise, require a bit more time and skill level.