Also, if you can’t figure out who to sic the robber on, place it on the person to your right.
It probably goes without saying, but robber placement should hinder the biggest threat to you and in general, experienced Catan gamers will be understanding if that’s what you do. Less forgiving is when you use it early on, hitting many people at once. That tends to leave a bad taste in people’s mouths as they look for retribution. Having multiple people gunning for you is not fun. Also, if you can’t figure out who to sic the robber on, place it on the person to your right. There’s a high chance it’ll stay there longer.