There is a carpark by Rauðfelsdsgjá, just a short walk away from the narrow opening. You can follow a path up to it, and walk in along a narrow path along a stream.

A little bit of clambering will get you to a small waterfall. Any who wish to climb in further than this will need to be reasonably fit, equipped with waterproof clothing and sturdy shoes, and willing to get pretty wet. This route should not be attempted if the ground is icy 💦