install older OS snow leopard on external HD

could someone please post simple instructions to install older version of OS on external hd. issue 2011, I bought Mac book for one reason that was to use for a recording studio. around the same time period I had purchase Pro Tools 9 and was auth upgrade to Pro tools 10. I deployed for army repeatedly, never had the time at home to get pro tools running and of course apple kept pushing out updated version of OS, Mac, pro tools and digital interface, just sat, couple of grand wasted. Ive seen some people explain it and tried to follow one of my biggest problems besiides compatable issue is PTSD for a few years now I couldn't comprehend what I read or fill out basic forms, or write weekly/monthly reports, I was force to retire. Docs told me my brain went into basically a SAFE MODE to protect it from any more damage. its been a few years now and much of it has come back, short term memory is still a little to be desired but... if anyone can give me straight forward sequence of haw to install older OS on EX hd and operated it I would be very thankful, most all I have now is music and playing my guitar for sanity. I have a copy of snow leopard, the external hard drive for dedicated installation and recording tracks my Mac is: MacBook Pro (15-inch, Mid 2012), processor: 2.6 GHz Quad-Core Intel Core i7, memory, 8 GB 1600 MHz DDR, graphics ntel HD Graphics 4000 1536 MB running Catalina10.15.7. help doing this would be so great, what's the old guy say in the commercial vets aren't looking for a hand out just a little help up. thank you,

Derek Colton