I’ve just done this, specifically replacing optical drive with an SSD and replacing the original HDD with a larger one.
I used time machine to do a full backup to external HDD, swapped both the drives and then just switched the MacBook back on. You select a wifi network, it downloads the OS and then you plug the external HDD in and select the backup you wish to restore from (i.e. the most recent) and, in my case, which of the 2 drives I want to install it on.
Could not have been easier really. I actually put the user settings and applications on the SSD first and then put all the stored files on the HDD.
Well worth having a bash yourself – worst case scenario, if you balls it up you just need to put the original hdd back in!