He/she must be very young!
This link displays the different types of nitroglycerin topical, including creams.
The explanation on how to use it is on this page as well, and the directions for nitroglycerin ointment are as follows:
For Angina: 15 to 30 mg (about one to two inches squeezed from tube) every 6 to 8 hours.
Nitroglycerin is more commonly used "these days" in a pre-made patch, were the messy cream and paper is no longer required. But, we do still this used. It can be adjusted more easily without having to get a completely different type of patch, so that can be an advantage.
So, your doctor will find detailed information on the link I gave above, as well as this will be in a PDR, which usually every doctor has.
Usually it is simply written: nitroglycerin cream, (and then the specific directions).
Again, it is available in any pharmacy.