I am no expert, but I'll post what works for me personally, other people might have a different opinion. So please keep that in mind.
When it comes to store bought juices or simple diy's, that have a specific flavor already but are very mild or lacking some of the advertised sub nuances, mostly when it comes to custards or cakes and creams, I'll fill 5-15 ml in a separate bottle and put it for 2-6 hours through my ultrasonic cleanser.
But you can do it with hot water bath too, fill some in a glass bottle or use a plastic bottle you know won't dodge when heated lol. Grab a pot/pan/bowl whatever and heat some water. Do not exceed 140F or your nicotine starts to fade, very noticeable if you do a couple of steep runs that day. Anyways, fill that water into a container, mason jar, bowl or anything you can seal, place your bottles in it and check every 30-45 minutes. If the water cooled down by a lot exchange, rinse and repeat. In some cases that can be sooner than my indicated time above.
Do that for 2-4 hours and you're set. Everytime you exchange the water, shake your bottle but don't open it. After the last round of the water bath, shake and open, let stand for 5 minutes not longer, then reseal and shake again. Now you can either use it, or put it in a dark place and let it steep more, but if you're planning on doing it via time anyways, then there's no reason to use the quick steep method to begin with lol.
Things to look out for or to mention:
If you have time on your hands and not desperately on your last drop of juice, steep it regularly aka dark place and wait 2-8 weeks.
Do not steep a whole 30/60 or 120 ml bottle of juice via quick steep at once, the result will be not very satisfying.
Use a bottle that you know won't crack/melt etc.
Do not soak the bottle tops in water. If you don't have a container small enough put your bottles into water repellent zip lock bags and then place them in the water. Last thing you want is getting your juice mixed with water lol
This method does not work very well with tobacco flavors or juices that are not potent enough by itself, custards can be tricky as well, especially if you have some multi layered custard, this method is just to bring out flavor profiles already existing in your juice, but not fully developed yet due to the store not pre steeping or made fully fresh etc. If your juice has barely or none flavor, this is not going to fix it, but it helps if you want to test a new juice or diy, or need to see were its at, for you to determent how long it would have to steep via dark place and time.
I hope it helps or makes somewhat sense lol.