This article will teach you the basics on how to install any Minecraft Modpack on your server, this article is applicable to all modpacks, however for example sake, we're going to use "Better Minecraft"
Step 1:
Click the "Minecraft Modpacks" button, this should take you to a list of all available modpacks.
Step 2:
You should now be on the list of modpacks, from here, type "Better Minecraft" into the search box, or; the desired modpack you wish to install.
Step 3:
Scroll down till you find the modpack you wish to install, then press the "install" button. You should be prompted by a wizard with recommended options, for simplicity, just follow what's recommended if you don't know what to do.