How do I lock iPhone 5 in landscape mode? I see how to lock it in portrait mode, and I've tried locking it in landscape mode using the assistive touch feature, but that isn't working for me.
You cannot. The iPhone is not like the iPad, which truly and fully supports the two modes (even SpringBoard rotates). The iPhone doesn't technically have a true landscape mode. At best, most apps support the orientation but that's as far as it goes. Rotation Lock on the iPhone locks the device in portrait mode. You can test this yourself by going into Mail, turning the device sideways (landscape) and then engaging the orientation lock. You'll see that Mail will orient to portrait.