Often overlooked, Apple Loops can be a hugely useful composition tool. So why not make your own in Logic?
As we haven't touched on this subject in quite a while I felt that it warranted revisiting, as Apple Loops can be incredibly useful, even to the more serious composer. There's a tendency to think that Apple Loops are simply a way for non-musicians to create cut-and-paste music, but that would be to underestimate their usefulness. The great thing about them is that their tempo and/or musical key can be adjusted over a surprisingly wide range, without incurring too many undesirable audible artifacts, and that their tempos conform automatically to the song tempo. You can also create MIDI loops to save in the Loop Library, so that their tempo and transpose value is taken care of automatically when you change those of the song, or use the global transpose track.
Why Make Your Own?
For the composer, then, these attributes make it very easy to try out new ideas. It becomes all the more apparent how useful this is when you create your own Apple Loops from segments of your own recordings. By building up a library of your own, you can combine elements from different recording sessions, forcing them to the same tempo and musical key. After that, it's up to you whether you use them as part of your final composition or re-record the composite musical parts in real time once you've decided what works.
Setting Up
The process of making an Apple Loop starts with creating an audio region that corresponds to the length of the loop you need: typically one, two or four bars. The region must be an exact number of beats, so setting up a loop marker and then using the Region Split menu to split the track by markers is an easy way to ensure you grab the precise length of region you need. 
Using markers to split your region to precisely the length that you want it to be is usually perfectly straightforward, but you may run into problems if you've used Stretch or Flex Time.
I've noticed that if you use Flex Time or Stretch to create a region, it may not be recognised as being the right length, in which case the option to create an Apple Loop will remain greyed out when you try to select it. It's thus often best to edit a longer section than you'll ultimately need, then use the Split command to separate out the desired region length when all other tweaking (and bouncing or gluing to consolidate edits, where that's necessary) has been done.
Note that while you can create an Apple Loop with the track effects applied, I wouldn't recommend doing this with delay or reverb, as the transitions between different loops may then become too obvious. You can add reverb and delay to the final edited track for the most natural-sounding result.
One you have your region ready to capture, you should make any level adjustments before turning it into an Apple Loop. Apple Loops may be mono or stereo, so set the track to mono or stereo as appropriate before making the conversion.
Make It Happen
The topmost item in the Region menu is where you get invited to add your region to the Apple Loops library. Once you select this, you'll see a dialogue box asking you whether the musical scale is major or minor, or whatever. It will also ask whether the sound is a one-shot sound that should ignore automatic transpose and tempo commands, or whether it's a loop that should follow these commands.
The Add Region To Apple Loops Library dialogue box allows you to categorise your loop appropriately so that you can easily locate it when browsing. Here you can also add key and tempo information.
If both options are greyed out, it means that your file is of the wrong length (not a whole number of beats) or that Logic didn't approve of your edits. If you edit before trimming, as I suggested, you should be OK, but if you trim first, then edit, Logic sometimes gets stroppy!
The conversion process is pretty sophisticated, as Logic detects transients and adds metadata that allows time and pitch manipulation to be carried out with the minimum of side-effects — which is why Apple Loops are so useful! You can then check the various category boxes on the right to determine how your loop will be presented. For example, it might be categorised as guitar, acoustic, clean, and 'single notes', with an added genre description, such as 'relaxed'. Pick a name that best describes the loop, and you're ready to save it. Logic takes a few moments to create the Apple Loop file, after which it shows up in the Loops browser along with all the ready-made stuff that's in there. Now you're back in drag-and-drop heaven, but with your own original material!
