Hi, most libraries will routinely ask for cut-down versions when you have songs accepted by them (although the specific lengths they want can vary), so its something you'll need to get used to creating. After a while you'll find strategies that work for you and it gets easier. Here's a few things that I might do, and this is in the DAW multi-track mix session (not the stereo mastering session)....
- Add a ruler in the DAW that's exactly 30s long (or whatever the max length needs to be, and start with the longest one), so you can clearly see the length you're aiming for.
- Slide that ruler until the right end is at the end point of your track (where the silence starts).
- Check where the left end lands. In an ideal world it might line up with the start of of a bar and musical phrase, but that hardly ever happens.
- Find the nearest acceptable start point to the left end of the ruler.
- If its close to the left of the ruler check whether you could slide the ruler until its exactly at the start point, then adjust the length of the fade-out on the ring-out at the right end (or perhaps shorten the final phrase to get a better fit).
- If not, find the nearest start point inside left end, then find a way to fill the gap between that left end and the start point. That filling might be a drum fill, or part of one, or a few solo musical notes (that might be there anyway), or even a backward cymbal or snare hit. Again, you might still need to slide the ruler around a bit to get the best fit then adjust the right end accordingly.
- If things get really messy because the chord sequences and/or phrases don't work, try copying in a phrase (or part phrase) from an earlier section that might work better.
- If all else fails, there might be a 30s (or whatever) section earlier in the track that might work as a stand-alone cut-down (but its very rare I've had to resort to that).
- Having found the section you want to work with, lock the ruler position.
- Be sure to mute all notes to the left of the ruler so you've got nothing hanging over into the start (easy with midi but audio can demand a bit more care).
- Once you're happy with everything, render the whole thing to the length of the ruler only,
- Check the rendered file length to make sure its correct and also ensure there's no hang-over or chopped notes at the start and the end.... DONE!
- Repeat the whole process again for the 15s version (or whatever the next shortest length is).
Ultimately, you might have to get creative to make your cut-downs sound natural, and some will be easier than others, but there's always a way. Actually, I quite enjoy making them, its a bit like solving puzzles
.