Hi grunty,

When you have Serato DJ Intro open, you should be able to look down the bottom left hand corner, there is a button next to "album art" which is a crate with plus sign.

When you click this it will create a new crate in your crate directory, and to make this new crate a sub crate of another, simply just drag and drop that crate on another one.

It will now appear as a sub crate and the parent crate will have a drop down arrow allowing you to expand it.

I hope this make sense, let me know if you have any further trouble :)