I have a 2015 Mac mini that I upgraded to Mojave. It runs but very slowly. I'd like to go back to Snow Leopard, which will also get me back fax modem support. There's nothing on the mini so I just need to wipe the drive. I have the Install DVDs. But I apparently need to make a boot device (thumb drive, etc.) to wipe it back to the old file structure. Any advice would be appreciated. TIA.