Vaginal moisturizers can typically be applied in two ways: either with your fingertip in the vagina and externally as needed, or with an applicator in the vagina. Applicators often come pre-filled (like the Yes product offered at MiddlesexMD) for one use, but, in general, the amount to use is an individual decision based on the degree of dryness or discomfort.
Manufacturers of vaginal moisturizers generally recommend application every 3 or 4 days, but, again, it's you who will need to find the frequency that works best for you.
Women I talk to often like the pre-filled applicators for "maintenance" application, and then to use a bulk bottle when they need lubricant for sexual intimacy. And, of course, you might find that a personal lubricant in addition to regular moisturizing is what you like best.