Reach in and if it was a normal presentation you'd feel 2 right side up hooves, a head behind(superman). If that was a water bag in the pic and she's been trying to calve since last Wednesday you're probably going to find something different. Tangled up twins, upside-down hooves (backwards calf), etc. Position the calf so you have hooves coming first, put calving chains around the legs and pull the calf out.
Here's a lecture - not that I haven't done the same as you briefly to some extent. You need facilities to have cattle. A headgate or a place to load them and take them to headgate. In the birthing process things should be measured in hours or minutes not days. Get a book about raising cattle, read it and have on hand. I bought Storeys Guide to Raising Beef Cattle but I'm sure there's others. Thing is it's the cattle that pay for you being unprepared and not knowing what to do, sometimes they pay with their lives. So if you're going to raise cattle you need to have the equipment and facilities needed to care for them.
All that said I hope that wasn't a water bag and despite the signs of a bad outcome you end up with a live healthy pair but still learn a lesson.