Firstly, make sure the rope you are using is strong enough to hold the weight of a person. Since you are going to be using the Munter Hitch knot for belaying, the rope needs to be able to withstand weight.
Make a simple loop in the rope, about the diameter of your index finger and slip it on the open carbiner. This loop of the rope is supposed to be one carrying all the load, so make sure it is close to the carbiner’s spine for better support.
Now create a second loop, with the rope coming on from the opposite side of the first loop. This loop also needs to be slipped onto the carbiner. Now you will have two separate loops on the carbiner, with the one near the spine holding the weight.
Pull ends of the rope to tighten the loops around the carbiner and pull on them to ensure the loops are securely settled down.
Now you need to lock the carbiner properly. This is very important to ensure the safety of the people who are going to be hanging on to one end of the knot. Do not risk lives by using a Munter Hitch knot with an open carbiner.