First off if the cap is off on the juice; this is known as breathing and all your doing is thinning your juice out and over oxydising your e juice. True steeping is capping the bottle, putting it in a dark area (box) for a period of time.
Now my over all recommendation when it comes to steeping and gaining the most out of your flavor is this.
Know what kind of juice your dealing with, fruity, custard, fruity and custard, etc.
With fruity only based juices there's not much work you need to do for these most are good just right out of the bottle, but if I were you with these shake the bottle for a good 15-20 seconds, take the cap off let it breathe for around 2 - 6 hours, but no more than 12 at any given time. Then cap the bottle, put it in the dark area again, for a day take it out shake it, un cap it let it breathe for 2 hours, re cap it let it steep for two more days now. Juice should be good by then.
Now custards, there's more to it. Personally, custard juices are thicker and tend to take more time to gain the most flavor out of them. If I were you first thing I'd do is shake the bottle for a good 30 seconds, un cap the bottle let it sit in dark area for around 4-6 hours, take it out re cap it and shake again for another 30 seconds. Let is sit in the dark area now for 48 hours. Take the bottle out, If it's a squeeze bottle just pump air in and out of it; this method is something called Streathing, it will add oxygen to your juice and break down the Vegetable Glycerin into the PG creating more flavor essentially. Now once you basically pump air into the juice let it sit in the dark area for about 2 hours un capped, re cap this run under hot water for 10 mins no more then that, then cap it put in dark area for 3-5 days let it sit there and dont touch it. This should be around 7 days after. By now if you've done all these steps the juice should be better, but if it's not simply repeat the process and do it for an addition 3-5days+. Remember custards take more time than just a fruity juice.
If it's a fruity custard, basically do the same thing as you would for a custard, but less time for steeping.