Your first step to repairing your leak is to make sure the car is switched off and cools down completely. This helps to remove any pressure. Removing the radiator entirely can help you to work on it better.
Try to locate the leak first, and then drain the radiator until just below the crack or damaged area. Clean the area using brake cleaner. The surface must be completely clean and dry before applying the epoxy.
Find epoxy that comes in strips, break a piece off that you will need, press, and rub together until it becomes softer. The more you squeeze and work with it, the more pliable it becomes, which makes it easy to work with.
Apply the epoxy and cover the entire area, any surplus can be removed. Take your time when doing this, the layer should be correctly applied so that when pressure builds inside the radiator, the epoxy will remain stable.