Requirements:

A FreeMC.Host Minecraft Server. Create one here for free

Step 1: Open the ViaVersion Spigot page

Link: https://www.spigotmc.org/resources/viaversion.19254/ This is the plugin that allows multiple versions to connect to your server.

Step 2: Click Download on the top right

After a few seconds (but can take up to a minute), the plugin will download from Spigot's website to your Downloads folder

Step 3: Open the panel

This is located at panel.freemc.host - you should be logged in automatically with your account

Step 4: Click on the server

These are shown in a list

Step 5: Select File Management on the left

Step 6: Click on the plugins folder

If it doesn't exist, start your server and it should automatically create

Step 7: Drag the plugin to the middle of the page and wait for it to turn green

Step 8: Go back to the console and restart your server

Open your minecraft client and check if it works