You should always signal to exit a roundabout after you pass the exit before the one you want to take. If you want to take the first exit, signal before you enter the roundabout. If you want to take the second exit, signal after you pass the first exit etc.
If you’re taking an exit more than 180 degrees around the circle from the entrance, you should signal the other direction to show that you are staying on the roundabout and then signal to leave after you pass the exit before the one you want. E.g. if you’re in a country where you drive on the right and the roundabout is on a traditional crossroad ‘+’ and you want to take the third (left) or fourth (u turn) exit then you should indicate left to start with and then indicate right to exit.
Note that this is what you should do. You will see drivers not signalling correctly in any country with roundabouts. The most common error is probably not indicating at all when going straight.