I don't know where they're at, but there have been some extensive reports on how people collected trees.
First things first: be in the right season. It seems that for junipers, this usually is early spring.
In a nutshell, and basically with some plant knowledge, this is really all you need to know:
Collecting is about keeping as much foliage as possible and keeping the rootball intact. Don't let the tree dry out in the sun, take it home with as little disturbance as possible. Then get it into a good (wooden) grow box and fill it up with good bonsai soil. I made a few boxes on the fly, but honestly it's better to go out and scout your tree, check how the root spread is and make a fitting box before you start digging. Larger is better sometimes, smaller is more handy and easier to control. Once you've got it in the pot, it's about providing the right after care. This right after care can take two to four years.
Some people swear by cleaning out the rootball in one go, others give it a few years in a pot before removing the old (field) soil. I've had mixed results with both in pines, as well as junipers. I depends on how the soil is and how it acts next to the bonsai soil. Sandy soils for instance, easily wash out and are almost automatically replaced by sinking bonsai soil. More loamy soils can hold way too much water. It depends..
Some people swear by misting their trees after collection, I don't. Nature holds a lot of pathogens, and I'm not going to give them the advantage on a weak tree by misting. Others might think that getting the tree healthy first is better, because a healthy tree rejects pathogens on its own. I can support both arguments! But not misting is easier ;-)
I'm a fan of supplemental sugars, others think that's a bad idea. I feel comfortable saying I did over 450.000 plant transfers onto carbohydrate rich media, and all of them made it. But that was in a laboratory, not in a pot.
See? There's various approaches and they're all right from a certain point of view.
It all comes down to the fact that it's basically a very, very, very rude/crude repotting. It can take a few months before the damage shows, and knowing what you're doing is the key to success. If the specimen in question is really worth collecting, then give a few non-worthy trees a try first. This'll give you a bucket load of experience. Knowing what type of tree you're collecting and how others have collected the same type can also learn you a great deal.