"敷以膏药" can be translated into English as "apply a plaster (or ointment)" or "dress the wound with a plaster", depending on the context.
If you're emphasizing the action of applying the plaster to the affected area, "apply a plaster" would be appropriate.
If you want to highlight the process of treating or covering the wound with the plaster, "dress the wound with a plaster" could be used.
In most casual or medical contexts, "apply a plaster" is a common and straightforward expression.