It may depend on the OS, but I’m on Linux (Ubuntu), and it couldn’t be easier. I have a small “+” on the right side of the URL bar (just before the “favorite” icon). Clic on this, and it installs it. It changes to a “open in Discourse Meta” icon when installed. If I uninstall the PWA, the “+” comes back to allow to install it again.
And if I go into Chrome’s menu while surfing meta here, I have similarly a line “Install Discourse Meta”, which becomes “Open in Discourse Meta” once installed, and becomes again “install…” if I uninstall.
So, one icon in the URL bar + one line in the menu. At least, it’s like that for me. I wouldn’t know what to ask more 
You don’t have to rely on the popup message at all.