CIM stands for cum in mouth, where you ejaculate into your partner's mouth. The key difference between CIM and CIM (with condom) is, as you've guessed it, that you are wearing a condom when this happens. Not only do you still get your happy ending, but you also get to be safe in the knowledge that it is all safe, as the condom stops the risk of infection. This means safer and happier sex for you and your partner.
There are such a wide variety of condoms that a simple plain one might not be needed. Some prefer the taste of flavoured condoms, which can help the person performing as they don't have to worry about the taste of the latex in their mouth and can concentrate totally on pleasing you. It is best to remember that, while flavoured condoms are a great answer, the flavours can fade quickly, and so it is best to have some spares to hand in case they need to change it.
CIM (with condom) usually comes after oral (with condom). The beauty of this is that you won't actually have to change anything you are doing - you can just finish where you are without worrying about changing positions. All you have to do is sit back, relax, and enjoy the moment!
That said, you may wish to talk with your partner while it is going on. Let them know know feels good as they use their mouth on you, and while there is a condom there to prevent your cum going into their mouth, a quick "I'm cumming!" can be a great ego-boost for them and let them know they've done something really right. A gentleman always gives their partner warning when they are about to blow their load, so just let them know so they can prepare appropriately. Not only will your partner be thankful, but they will also be more likely to do it again in the future.