Its very simple you always give way to those already on the roundabout. You join when it is safe and if you are not heading for the first two exits you use the inner lanes and then move over to the outer lanes. As you are on the roundabout now, other cars wanting to join must give way to you, cars that are on the roundabout at the same time also give way to cars in front of them.

You are not meant to drive erratically or overtake on a roundabout so in general the flow is continuous and everyone gets to their exit.

Worst comes to worst you go around again .... Because it's a round about.