In some cases, I seal the outside. In other cases, I seal the inside. On some, I do both.
If you seal the outside, more sealant is likely to leak out and be visible, which is not so good. If you seal the inside, it won't be visible. If you seal only the inside, and if enough rainwater can seep into the seam, it will be caught there. That might lead to a mold problem.
In some cases, I have just lightly smeared the stuff on. That isn't very neat at all. In some cases, I have used a plastic syringe to inject the sealant more directly into the middle of the seam. That works good. In some cases, I have just applied dots of sealant into the needle holes.