Install a Distant Horizons & Iris Shaders server through the Control Panel (Servers → Select your server → Game servers → Add Game Server → Distant Horizons & Iris Shaders)
Experience has been great! ran into a couple of problems with mod compatability but the customer support team was very responsive, friendly and helpful. Couldn't completely solve some of the problems but found some very acceptable compromises, which is still very good. Would recommend!