I don't think that any one method of taking Essences is superior to another - some people are so sensitive to the vibration they only need the Essence to be within their energy field, in a pocket for example, and they can feel the effects. You must trust what you feel and go with that, ultimately it will be what you need.
But, as you asked here is what I can offer by way of information to guide you:
One drop once a day is a high dose, rather like homeopathy, where the more diluted a remedy is, the more powerful it is. So, if you take a dose and find you are reeling from it, then take more and more often.
You can take the Essence directly onto or under your tongue. You can put it into a small amount of water or into a whole bottle full - though I had a client who put three drops into her water bottle and then had the overwhelming urge to drink all 16oz down in one go. I do both; under my tongue sometimes and in my water bottle at others during the day. I also add essences to my body lotion, just a drop or two and I have put them into my bath as I am inspired.
You may experience a 'reaction'. Such as is the case with many vibrational healing methods - Reiki for example - where you may develop a cold or becoming very tired. You may feel dizzy in the beginning as your body assimilates this new vibration, none of these (or any other reactions) need worry you, deal with the cold as you would normally, sleep if fatigued and in the case of dizziness, which usually passes quickly, you may want to take more doses more often until you feel more balanced.
If you experience what you perceive to be an adverse reaction, then of course, stop taking the Essence. I have never heard of a case where a Plant Spirit Essence has caused such a strong reaction or been contraindicated for any pre-exisiting ailments, but I have to add this commonsense note here.
Finally, as I am not given to re-inventing the wheel, you may want to look at dosage information and FAQ on the flower essence sites listed in 'Further Information'.