1: Upload your downloaded zip to /var/games/minecraft/import/ (which you’ve already done)
2: Use the Web UI to Import that uploaded zip (Which you’ve also already done)
3: Go to Profiles in the WebUI and download Mojang 1.12.2
4: Head back over to the server status page and change profile to mojang 1.12.2 then click Copy profile to live server files
5: Change runnable jar to forge-1.12.2-14.23.5.2768-universal followed by changing your -Xmx value from 256 to 4096
6: Accept the Minecraft/Mojang EULA if you haven’t already, and start your server.
Keep in mind that the first start might take awhile depending upon your hardware (for me it took about 12 minutes before it was ready, though sequential restarts afterwards should not take as long as the first). If you’ve got better hardware than I do then it’ll be much quicker for you than it was for me.