question of dosage calculation using ratio and proportion method

I need help with a dosage calculation. I am not asking necessarily for the answer but more a guide on how to get there rather. Here is my problem:
The physician's order reads-
Atropine sulfate 1/150 grain IM pre-operatively: on hand is atropine sulfate 1 mg/mL. Using the ratio and proportion method, how much atropine sulfate should the health care practitioner administer.

I really appreciate any help I can get regarding medical math because I take nothing but online classes, due to a lack of transportation, but I flourish in the classroom where I am shown where to start and how to figure a problem out, so I am having a little trouble.
Thank you,
K