When growing a bonsai trunk one needs to find the best way to grow a trunk fast. As discussed in why
The general idea
If you have a plant that you would like to use as bonsai, but it is a tall tree that has been grown for use in a garden or forest plantation, it is not a lost cause. This is exactly what you are after with the grow and cut method: You leave the plant alone for a few years, untill the trunk gets close to the desired girth. Then you cut the tree down to a stump and let it grow out again. This process is repeated a few times untill a naturally tapered trunk is obtained. Make these cuts in late winter, when the tree is not growing. This way, all the energy stored in the roots can be used to grow a new leader. In general it takes at least three cuts to obtain a naturaly looking trunk.
Ratio’s and sizes
Typically, one would let the plant grow untill about two thirds of the desired trunk thickness has been obtained. That is the point at which you cut the trunk at one third of the desired final tree height. You can estimate the final tree height quite easily, taking into account the 1:6 to 1:12 trunk thickness to tree height ratio (read more about this). The next segment of the trunk you would let grow untill it is about two thirds of the trunk just below the cut. You cut this off at about one third of the height to be grown about the previous cut. Repeat this step at least three times. After each cut, you can expect a flush of growth, and many sprouts appear. Select the strongest sprout, or the one that is in the best spot. With a tiny wire place it up, in a natural curve to the original trunk. All other sprouts are removed, in order to focus all the trees energy to this one sprout.
When a plant is cut, the bark below the cut will die off. Many books advice to make a slanted cut to facilitate this. However, it is not always certain how the bark will die-back and which branches below the cut will survive. By making a slanting cut, the cut is much bigger than when you would cut just straight through. This increases the risk of infection and drying out. I am therefor a fan of cutting the trunk straight, and letting the bark die-back naturally. A few months after making the cut (mid growing season) you can see the die-back line in the bark. At that point you can take away the died part of the trunk with a dremel or saw.
why it works
When one looks at a tree with a big straight trunk, one often thinks: This is very bad material for bonsai. This is however not true. By cutting low on the trunk, one forces a flush of new growth from which new leader can be selected. The new growth will add girth to the trunk below, and the bark will fairly quickly grow over the area that has been cut. As such, large wounds are covered. By the time the second cut is made, the largest part of the first cut is already covered with bark.
After each cut, one gets a slight bend in the trunk, exactly what one wants for an informal upright style. So do not worry too much about how the new leader is attached to the main trunk. The slight bends in the end will look natural, unless the branch is at a straight angle to the original trunk.