Find the .zip file from step 1 and double click it to select it.
You may get an error here. If it's ID: -1 or any other error, refresh the page and try again. If that doesn't work, try a different browser like Microsoft Edge.
Once the world is uploaded, look to the left of your screen. Go to the "Files" section.
Go to the server.properties section and scroll down to resourcepacks. Paste the following link that corresponds to the player limit of the world you downloaded.
10 Players: http://resourcepack.host/dl/6OYRoGeXyU7EdAgEtK9gjnQlhUp3H0DY/USWRP-10-PLAYERS.zip
25 Players: http://resourcepack.host/dl/nIJPWxlIiEJxE8aEPTaNqbeQhFEU4LLq/USWRP-25-PLAYERS.zip
Look to the left of the resourcepack field. There should be a save button. Click it, and if you see a green check mark, you're all good. If you see a red X, then you probably incorrectly pasted the link incorrectly. Try it again, or try refreshing the page. If neither of those work, use a different browser or use the other link, ex. if you used the 10 player limit link then use the 25 player one.
Once you've gotten the green check mark, then you've done everything properly and you can now join the server! Launch minecraft 1.19.2, go to multiplayer and input the IP for your aternos server.
Enjoy playing!