In the creeperhost website, click the "order" green button on the on the picture that says "Modded Minecraft".
Step 4: Entered any name for your server (type anything, its just a label). Next, dragged the green bar to the very right until it says "custom players". This will unlock a hidden button on the right. Click this hidden button called "Select Package."
Step 5: Now choose my server hardware. I would recommend minimum of 6GB for 4 people. Note: For comparison, if you're setting up a vanilla server, a minimum of 1GB for 4 people is enough.
Step 6: Next, find the modpack you want them to install. You can check out the list of modpacks here on a separate tab: https://minecraft.curseforge.com/modpacks. In this example, I am going to install a modpack called Roguelike Adventure. It is an adventure/exploring modpack. Here, I type the word "Roguelike" and it will give me a long list of versions. I chose the one on the very top of the results since that is the latest version (picture maybe outdated).
Step 7: At the bottom of this website, fill out the form. This website takes credit card or paypal.
Step 9: This is the Control panel of the server. You want to click on "Create Instance" to start it up. It will ask you for name/label for it on your control panel, type anything. Once you hit okay, give it a few minutes to set up. The picture below took me 3 minutes to start it up. For the modded minecraft server, the initial startup load is probably going to be 10+ minutes. It may even crash. If that happens, click on the orange button on the top right to restart it. You may see a lot of errors in the console while its loading. This is normal. Finally, the IP address is the one below the name you gave it. For example my ip address is "QueenGambit.playat.ch"