Once you've got hold of the w7 media, must be same version as you had eg same as what your key is for.
Reboot PC and enter BIOS, change boot order according to whether w7 is on bootable USB or DVD.
Save BIOS changes and reboot.
Follow w7 install, you can select the same hdd that currently has w10 on but choose to delete it's partitions so it shows one lump of unallocated space and hit continue.
At some point it will ask for a product key so enter your w7 key.
When you're back into w7 let it do some of the upgrades, I think it needs to have at least upgraded to service pack 1 (SP1) and needs to have activated before doing the in place upgrade to w10.
Once it upgrades to w10 you also need to wait until it has activated and then if you want you can then do a fresh install (custom install I think is how it is termed in the installer, 2nd option down).