Step 1: Do a complete backup of both your SDD and HDD to external media. If the Ubuntu installation goes wrong, it can wipe both hard drives. I strongly recommend doing a full-drive backup to external drives that are the same size, or at least making images of the entire drives to a single large external drive. A free utility such as Clonezilla Live can be useful for that. (How to put Clonezilla on a CD or USB drive.) However you do it, just make sure you have a really good backup, before you begin.
The biggest question you need to answer is how much space you want Windows to have left over. If you're OK with Windows only having ~100GB left after you install Ubuntu, then install on the SSD. You will need to allocate swap space roughly equal to the amount of RAM you have. (You can do less, but it's not recommended.) So, realistically, Ubuntu will take up 50GB+RAM on whichever hard drive you install it on.
Before you begin, disable "Fast Boot" in Windows, because it will cause problems on Ubuntu. (Do this before creating the backup.) Un-check the box that is indicated in these instructions to disable Fast Boot: https://lifehacker.com/enable-this-setting-to-make-windows-10-boot-up-faster-1743697169. Note that Windows Updates will auto-enable Fast Boot, from time to time, so you'll have to periodically repeat this process, or Ubuntu may fail to boot, or be unable to write to/read from your Windows partition.
Next, install Ubuntu using the option to install alongside your other operating system (for installing on the SDD), or "something else" (if you want to install on the HDD). Please follow this guide for how to install Ubuntu: https://www.tecmint.com/install-ubuntu-16-04-alongside-with-windows-10-or-8-in-dual-boot/. I recommend booting the live CD/USB with the "Try Ubuntu Without Installing" option, so you can access additional tools while you do so (see below.) When you're ready to install, just double-click the appropriate icon on the Desktop.
Whichever drive you install Ubuntu on, make sure that you install the GRUB bootloader to the primary hard drive (the SDD). This will most likely show up as /dev/sda, but if you're not sure, open a terminal within the Ubuntu live environment and type in "lsblk", then press ENTER. Also, you can get clues as to which one it is by typing "sudo blkid" (ignore the partition number at the end of the device name, such as /dev/sda"1", since you want to install GRUB in the drive root, not the individual partitions). The Ubuntu installer might not need you to specify this, so if it doesn't give you the option, and boots correctly, don't worry about it. Be aware that Windows Updates and Windows Repair can delete the bootloader, so if you find that you can't boot Ubuntu, anymore, and Fast Boot is disabled in Windows, you'll need to reinstall GRUB using these instructions: https://help.ubuntu.com/community/Grub2/Installing#Reinstalling_GRUB_2.
