We use the Valhelsia 6 modpack and the server address is helixnebula.cc. There are 2 options I have tested that allow you to easily connect to the server. Both require a Microsoft account that has purchased Minecraft Java Edition.

Prism Launcher is an open source launcher that allows customization and profile management for Minecraft Java Edition. It is available on all major desktop platforms and can install and manage a Valhelsia 6 profile for you.

• Download and install the launcher from this link.
• Follow the prompts, which will walk you through logging into your account.
• Once you have arrived at the main menu, click Add Instance on the toolbar at the top of the window.
• On the list on the left side, select CurseForge.
• Enter Valhelsia 6 into the search box.
• Select Valhelsia 6 from the results, leave the version selector as default, and click Ok.
• The client will download everything. This may take a while. When finished, the profile will be selectable from the main menu.
• Right click the new profile and select Edit.
• On the left list, select Settings.
• Click the checkbox next to Memory and set the Maximum memory allocation to the highest value your computer can handle. This may require some tweaking. Failure to allocate more than 4GB of memory may yield a poor gameplay experience.
• Either click the Launch button on the bottom, or close editor and double click on the profile from the main menu.
• When the game is launched, enter multiplayer menu and add the helixnebula.cc server address.

If you have problems using Prism Launcher, you may try the official CurseForge app. I do not provide detailed setup instructions.