I'll agree with Jimmy on most points, but the 2 weeks to a couple of months steeping period is a little extreme for the vast majority of juices.
Generally, when you receive your new custom-mixed juices, merely shake them really well, then open the cap and set the bottle aside in a cool dark place as Jimmy instructed. Leave them that way for 1-2 days to allow any alcohol base to evaporate if present, then recap and shake well again. Let sit in the same cool dark place for another couple days. At that point, most flavors will be ready to vape.
Some flavors don't require steeping at all. A hint that they will is if the vendor custom mixes your juices to your specifications. Things like ratio of PG/Vg, extra flavor added, menthol added, etc. These often have a birth date, or creation date printed on the juice bottle.
Most non-custom mixed juices will just ask for nicotine strength. These are mixed and bottled way in advance of your order, and sitting on a shelf in a warehouse waiting to be shipped out.