Hello and welcome to the forum. Now is not really a good time of year to dig deciduous material. If you have some in place now leave them there and feed them well. Let them stay in place for several years while the trunks thicken up.
Another option would be to dig them next spring and relocate them to a better location where they can stay for several years. This gives you a chance to begin to work the roots. A trick you can use is to plant them on top of old tiles, plates or even boards. This helps to develop a shallow root system and will make the eventual transition to a bonsai pot easier.
It's a misconception to think that you need to start with small material and train it immediately as bonsai. If you take this route it will take much longer than if you just grew it out at first. It is not uncommon to spend years, or even a decade, simply developing a trunk.
There are several steps involved but the first few years are spent primarily just getting some bulk to the trunk.