It's really really easy. You just need to find out the IP of your Switch in your local network.
Then take the NSP, give it a short name and place it in a folder with
this python script (have Python installed obviously).
On your Switch launch tinfoil -> install NSP -> network install
On your PC -> CMD -> cd in the folder with the NSP -> run the python script -> enter Switch IP -> enter NSP name -> hit Enter (don't need to put in your PC IP) and it should start
Don't mind the errors and other messages as long as the installation on your Switch's screen keeps progressing.
When you're done, you have to completely exit Tinfoil (I'd even recommend to leave hbmenu and return to the homescreen for 1 - 2 seconds) then start Tinfoil again if you wish to install another NSP and start over. I experienced that installing another NSP from within Tinfoil right after the first one without exiting it, does not work.
Also a last small hint: Give your NSP a very short spaceless name like
mario.nsp and
marioupd to be 100 % sure.