+1 To vids here too.
Disassembling a gun isn't complicated by itself, it's more when it come to assemble it back.
That's why you take some notes about it.
For disassembling an Mp5 you may want to take a look here :
http://www.mechbox.com/mp5
There's also a vid on how to reassemble it.
But honestly, it's not that complicated, you just need to take your time, and remember carefully where each parts goes.