the most secure possible? you’ll need to learn a ton. you’ll get there, but it’ll take a while.

decently secure? install Linux Mint, install your updates, don’t run sketchy commands with URLs in them unless you know what you’re doing, maybe follow a hardening guide. you’ll be okay.

if you need to be extremely secure and private, install Tails on a USB stick. it will be slow and frustrating, and you’ll need to save files to a second USB drive, but it will probably keep you pretty safe, and it’s decently user-friendly. just make sure you keep Tails updated! you’ll have to do that by flashing the new Tails onto a new USB drive, there’s no easy way around that.

those are your two most user-friendly, safe approaches.

specifically this is how QUANTUMINSERT worked (from the Snowden leaks.) also China used the same technique, injecting malicious JS through the GFW to get bystanders to DDoS github, in a much more obvious and indiscriminate way.

nobody here is remotely likely to be targeted by NSA, of course, but you can actually do such attacks on a budget if you compromise any router in the chain. combined with a BGP hijack it’s not far out of reach for even a ransomware gang to pull something like that these days.

Zalgo captchas would go so hard.