IP spoof

Hello, I keep reading that an ip address is "too easy to spoof" and that is why it shouldn't be relied on for most things... So I am wondering, if it's so easy, how do you do it? How could I make a request to my document and make the access appear in the apache_log file as if it was from an IP address I specify? or is it not that easy after all? Anyway, I am curious how this would be done.. Thanks, ..rick