There are calculators that do this for you. I have collected the ones usable offline
here. I particularly like ddavelarsen's. To do it manually you just need to know how many total drops you're using and use the same dropper type so each drop is the same size or as similar as possible. Note that very thick or very thin liquids like pure VG or distilled water will have very different drop sizes and distort the end result.
You take total drops and multiply it by the % of each ingredient in decimal form.
Simple example:
100 drop recipe
10% flavoring = 0.10*100 = 10 drops
50% 50mg/ml PG = 0.50*100 = 50 drops
40% pure PG = 0.40*100 = 40 drops
Complex example:
60 drop recipe
5% flavoring = 0.05*60 = 3 drops
12% flavoring 2 = 0.12*60 = 7.2 drops = round to 7 drops
33% pure PG = 0.33*60 = 19.8 drops = round to 20 drops
50% 50mg/ml PG = 0.50*60 = 30 drops