Hi,
I'll take a crack at this. I am assuming your iTunes Library is in your old HDD, which was also your OS drive? If that is so, I would move/copy the music files in the iTunes folder to a temporary drive. Either a big enough USB stick or external drive. Otherwise, you will have 2 OS's running. You'll need to reformat the HDD, anyway.
Where iTunes app is installed is usually where your OS is installed. In this case, your new SSD.
This is what I would do, if you were me:
1) Copy the entire "iTunes" folder under "Music" in OS X to a big enough USB stick, or, external drive
2) Copy any other personal files from your old HDD that you don't want deleted forever
3) reformat HDD; via Disk Utility (I guess you can do this now or later before installing OS X; or afterwards; your pick)
4) Install SSD and HDD into your laptop
5) Install OS X into the SSD; (this is also the time before installing OS X to format the HDD if you choose disk utility from the menu)
6) Install/update your apps; (I guess you can also format that HDD at this time, if, you haven't done so)
7) copy/drag the "iTunes" folder you copied into your HDD in the optical bay (you definitely need to format that HDD before doing this step; google how to format HDD in OS X if you don't know)
8) as powerslave mentioned, open iTunes by clicking it and holding the Option Key, wherein, it will ask you to choose a Library or create a new one. Click Choose Library, find the iTunes library you had just copied and you're done.