How should I apply this ointment for rubbing tired muscles?

A little goes a long way with a 10% camphor concentration. Take a small amount — roughly a pea-sized dab per area is a sensible starting point — and rub it into intact skin over the spot that feels tired or strained. The anhydrous lanolin in the base gives the ointment a softer, emollient slip, which suits a slow massage-style application to areas like the elbows, knees, shoulders or lower back.

A few practical points:

• Rub in circular motions until the ointment is distributed evenly; because the base is dense, it takes a bit longer to work in than a light cream — that is normal and part of how it stays put.
• Keep it to specific areas rather than large surfaces of the body. This is an occasional, targeted product, not something intended for all-over use.
• Wash your hands thoroughly after applying. Camphor residue transferred to the eyes, nose or mouth is very unpleasant, so this step matters more here than with ordinary body creams.
• Do not apply near the eyes or mucous membranes, and avoid touching your face until your hands are clean.

There is no need to cover the area with anything afterwards — the petrolatum-paraffin film already keeps the camphor in place on its own.