I just installed a second hard drive in the place of the optical drive in my MacBook the other day! It's pretty nice having the extra capacity.
Even if you just replace your current hard drive with your SSD, you'll see a significant speedup.
Here are the relevant guides:
'''Replacing the current hard drive:'''
[guide|3401]
'''Removing the optical drive:'''
[guide|3418]
Go to the last page of this guide on replacing the optical drive (it's for my machine, but the last page would help for installing the second drive)
[guide|8516]
'''And finally, the part that iFixit sells for the second hard drive:'''
[product|IF107-080]
Good luck and enjoy your SSD!