Basically what ^^^ enigma27 said.
Download the 'Media Creation Tool' from MS and use it to download w10 onto your USB thumb drive. Choose the correct version to download, if your w7 is 64bit Ultimate or Pro then choose w10 Pro 64bit (non N version if you want the media apps).
Use 'Disk Management' in Windows to remove partitions from SSD, leave it as unallocated space.
If you want your system to be more resilient to a virus infection at start up then you need to set up secure boot. This requires a few things though, a GOP enabled VBIOS on the graphics card, GPT rather than MBR on the SSD (can set this in 'Disk Management' step above). After you've done the upgrade on the old hdd and w10 is activated and the old hdd has been disconnected whilst pc turned off, reboot to BIOS, change boot type in BIOS set to UEFI, boot to UEFI USB directly from BIOS to start the install process. After install complete, reboot into BIOS and enable secure boot and fast boot options. In Windows confirm in System Information that secure boot is recognised as being on.
After you've upgraded w7 to w10 on old hdd, shut down Windows / PC, then disconnect old hdd. Make sure in the BIOS it has SATA type set to AHCI. Ideally you'd have also downloaded the correct Intel Rapid Storage Technology driver and copied it on to the USB too as you can provide this during the install when it's asking you about which drive to put Windows on to.
If not doing the secure boot method then put USB thumb drive in a free USB port and boot up, without the old hdd connected it should boot from the USB thumb drive.
During the install process running from the USB, choose the custom option which will give you a clean install on to the SSD. Skip any requests to enter the product key. Create a local user when asked with whatever you use for a username. If you have a MS account you can change the local user over to a MS sign in type after install has finished (this gives a better result for your user folder).
Let MS download and apply updates, safely remove USB and reboot.
Install various drivers obtained for your motherboard from their website if they're newer. Check graphics driver version against latest available and upgrade as appropriate.
Shut down, reconnect old hdd, boot up, use 'Disk Management' to remove partitions from old hdd, set to GPT, then create new basic partition on the whole of the unallocated space. You can now use the old hdd as user data / doc storage, alternative steam library location.