In terms of the how, when you install Linux the graphical installer will allow you to specify how big the drives should be and where the main system partitions should go. Obviously back up your personal data from the device before you do this!!

I'd let the installer suggest a partition layout (telling it to remove existing windows partitions - back up any data you want to keep!!) and then edit whatever it suggests so that it makes swap and boot partitions for you. Leave the swap and boot partitions as they are. Reduce the OS / partition to 50gb to leave empty space on the SSD, and create a new ext4 partition on the HDD filling it and give that the mount point /home. To make space for the new hdd partition you may need to tell it to delete any existing partitions on the HDD drive if the installer doesn't (again ensure you've backed up shy data you want to keep!).

The spare space on the SSD will be for the games partition. It's probably easiest to leave that empty at first, install Linux and then once linux is set up boot in and use a partition manager to create the games partition in the empty space. The reason being you will need to specify a "mount point" for Linux to use for that drive - a folder basically - and it won't exist at the time of install so may be a headache to sort at that stage as a newbie.

So complete the install and boot into your new Linux install and then I'd make a folder in /mnt called games. Easiest way is open a terminal and type:

sudo mkdir /mnt/games

It'll ask for your admin password, put it in and the folder will be made.

Then you can open a partition manager such as gparted. If gparted isn't installed then you can install it either from the Mint software store or open a terminal and put in:

sudo apt install gparted

This is using the admin account to use the software manager apt to install the packages for goarted. The software store does the same thing but is point and click.

Run gparted from your main "start" menu and it will show you your SSD and HDD. Select the SSD, select the empty space and add a partition.

Create an ext4 partition filling the empty space, give it a label like Games and then under mount point put /mnt/games

If there are any boxes for automount tick that so it's available at startup. I'm used to KDE and kparted which has this; gparted may need you to right click on the partition once it's created to set its properties for automounting.

Then restart the PC and you should be good to.go. The drive should appear in your file manager already mounted and ready for steam to use. I'd use lutris for gog games.