You ready to get your mind blown?

Soaking your jeans in water for anything over 40 minutes is a waste of time, the shrinking of the cotton fibers takes place because they've become wet then dry for the first time since being woven into a textile. How long you leave the jeans in the water does NOT equate to the amount of shrinking.
Actual test i've done just to make sure what i've learned is correct: Take a pair of unsanforized jeans and soak them in warm water for 25 minutes, take another pair of the same jeans and soak them for 2 hours. Difference in shrinkage? Pretty much unnoticeable. Technically? .1" in the waist, .2" in the length.

The temperature of the water makes a far larger difference. Kids who talk about "i soaked for three hours" are wasting 2 hours and 40 minutes of waiting time.

How do you soak jeans?
Soaking jeans is really pretty simple. You fill the tub with whatever temperature water you feel will give you the desired results, put the jeans in the water, and that's it. You can add soap if you want. Dr. Bronner's Castile Soap is popular for this purpose.

How long should I soak them for?

The most important thing is the temperature. If you're going for max shrinkage, you'll want to soak for around a half hour in warm to hot water. If you're trying to avoid shrinkage, use colder water and less time.

What do I do once I'm done soaking? Should I squeeze the water out, or just let them drip while hanging?

You can try to squeeze the water out, but the denim will have absorbed enough water that you wont be able to get it all out right away. If you can, hang the jeans by the waist in your shower to let them finish dripping.

If you're concerned that the jeans will be too small for you in the waist once they dry, you can put them on wet. Moving around in the wet denim will help it stretch back out. Just be prepared for whatever you're wearing to have indigo stains afterwards.

If you're concerned that the inseam will be too short after the wash, it is possible to stretch the inseam up to an inch and a half in some cases. To do this, lay the wet jeans flat on the ground, stand on the waist, and simply pull on the hem of the legs to stretch them. This is far from an ideal solution though, and you'd be better served doing a bit of research and buying jeans that are still going to be long enough after washing and/or hemming them.

Will I get different/better results from using a machine washer and dryer?

Machine drying will give you more shrinkage than air drying, so if that's what you need then by all means, machine dry.

The main advantage of machine washing is that you have a controlled temperature. You don't have to constantly monitor it like you would your tub to make sure the water stays at the right temperature.

Be aware that the tossing and turning of the jeans in the washer and dryer may lead to the development of odd creases and fades, especially if the jeans are allowed to sit crumpled up and wet inside the machine.

How much will the jeans shrink?

That depends on the jeans. Most raw denim will shrink between 1-3 inches in the waist and inseam. Some denim may take several washes to shrink down to its final size, so you should be careful if you're planning on hemming the jeans.

Also, some companies use sanforized denim, which will hardly shrink at all. Less than an inch all around, if that.

This is how you determine what size jeans to buy!
For Nudies:
Take a pair of your jeans that fits tight around your waist, button them all the way up, lay them flat on the ground, measure the waistband, double the number, take that number and subtract two numbers, THAT is the size you should buy in Nudies.

For Japanese Brand Jeans:
Take a pair of your jeans that fits tight around your waist, button them all the way up, lay them flat on the ground, measure the waistband, double the number, take that number and subtract one number from it, THAT is the size you should buy in Japanese brand jeans.

Hope this helps...again