dodgy plugin installation

hello,

i wanted to install a plugin, but it requires to add js and css links
first i added script tags to the header.php, but i been told not to do that

so i want to use wp_enqueue_script and wp_enqueue_style but i dont want to create a new theme and i dont want my changes to be erased when i update WP (or my theme)

so i found this tutorial
https://scribu.net/wordpress/optimal-script-loading.html
there is some php code but i cant figure out where to put the code itself (do i have to use child themes ?)

I actually installed WP so i did not had to put my hands on the code but i guess there ain’t no perfect plugin
and so far, I am completely confused about how to do that

plz help me out

appreciate