Not really hard at all. If you are doing an older small block chevy, the book 'HOW TO HOT ROD SMALL BLOCK CHEVYS' is a good place to start. Book has been in print for ages, seems like. Has all the basics in there, but obviously if working on a newer motor or different type you would need additional resources. I have always assembled my own motors after the machine work was done by a top speed shop. That way, you have some control over contaminants, torque settings and cost, etc. And, you have some satisfaction that you put it together, not to mention all the things you learn that help you after the installation. I'm not a big fan of 'crate' motors. They may have a warranty but who's to say that the person assembling your engine is not hung over from a night of bad beer and greasy chicken wings! I do however understand the convenience and attractiveness of the crate concept, for someone who can afford it or does not have time to build their own.