You could put the rosemary essential oil directly onto your skin.
However it's best to always dilute the rosemary essential oils before applying them to your skin to prevent irritation of your skin.
You can dilute the rosemary essential oil using a carrier oil such as olive oil which will prevent the rosemary oil from causing a rash on your skin.
When rosemary essential oils or any other essential oils are applied directly to your skin without dilution it's really too strong and some people with more sensitive skin have gotten burns from it.
So I would always recommend diluting it first and then apply the rosemary oil to your skin.