So first you need to know what the Linux designation is for each. I only have an HDD, so I can't say for sure, but I'd assume that the SSD would be named /dev/sda and the HDD would be /dev/sdb. Load gparted on a liveusb, it's a part of the installation of Linux Mint. From there it's up to how you want to set up. If it is a UEFI system then you want to have three partitions, one should be ~258MB formatted as FAT, that would be your ESP which is used for boot (I'm not 100% on how to set it up from scratch, so maybe someone with more experience can chime in). The second one should be 2x the size of your RAM and make it swap. The final one (I recommend) should be ext4 and mounted as / so that all files used for boot are on the SSD. The HDD should be ext4 and mounted as /home well that's how I would do it. It's a little more involved, so I'd check around the Linux Mint forums and Ubuntu forums as well. Also the Arch and Gentoo wikis may have some guides, though as they are different bases the guides won't work perfectly for you. Most of your files will be installed to /home, if you're up to some extra work you can also mount /usr as a separate partition on the HDD however it is a little harder in Linux than on any of the *BSDs or OS X, but the payoff is almost everything you install goes on your HDD and not your SSD.