The disadvantage to dripping is having to do it more often than putting on a refilled cart.
The advantage is a FAR better vaping experience.
WARNING: if you drip most cart experiences will seem BLAH afterwards!
PG is VERY thin so no need to dilute it with water, but if you wanted to use distilled water, not tap.
VG is very thick, so you may want to thin it out with PG, unless you cant use pg due to allergies. VG can be dripped straight or with a bit of flavor added also.
Best advice would be to get some of both, and try both straight via dripping 2 drops and vaping it to see the diference in them yourself.
Next try dripping 1 drop of each at the same time giving you a 50/50 ratio.
From here you will have a good idea what direction you want to go in when mixing your own. Flavors are harder as everyones tastes and atties vary, as well as the strength of the flavor itself. Starting low and working your way up is probably the best bet with most flavors.
Optionally you can just get some capellas flavors and vape them straight as they are allready PG based, or cut with vg or pg to your liking. They are a bit pricey to vape straight (still cheaper than most nic juices tho!), but I find I get great flavor only using about 25% capellas and the rest VG with most flavors I have tried from them.