How Do I Treat My Dog For A Moderate Allergic Reaction To An Insect (probably Flea) Bite?

If this is a sudden onset allergic reaction the best thing to do is take this dog to be examined by a veterinarian.  Injectable Benadryl and steroids if needed are the fastest and most effective treatment for an acute allergic reaction.  Benadryl will be followed up at home at a dose of 1 milligram per pound of body weight given orally three times a day for 2 days.  Do not use the Cold and Sinus variety only plain Benadryl is safe.
If this is a chronic problem with allergies she should be examined by your veterinarian so a long term treatment plan can be established.  Treating mild-moderate allergies involves anti-histamines, frequent bathing, food trial, omega-3 fatty acids, and treating secondary infections.  More serious allergies are treated with additional medication--steroids, allergy shots, and immune modulating medication.