There are three main types of Bonsai Trimming. Scaly plants, needle plants and leafy plants.
1: if the plant is scaly like a juniper, sequoia, chamecypress, thuja/cedar, or some heather, then you pinch the new buds as they are growing. One grasps the bud(s) which is longer then the mass which is present. One then snips the bud(s) with their fingernail. Try to snip the tips of any dead-tipped twigs too. Pinching will cause the plant to send out new growth, including new buds for you to pinch. Stop doing this is late Septemeber, start in late May/early June when the buds start to swell.
2: if the plant has needles that come from a growth candle (spike with no needles or leaves) or one that has a leaf or needle every couple millimeters, then one needs only snip the candle or branch at 1/3 its size when it slows down enlarging.
3: everything else has leaves. Rosemary, escallonia, boxwood, thyme, and fushia are some examples. Cut the growing branch off leaving at two leaves on the new branch, not including the leaf the branch came out of - if there is one. If the branch still sticks out a lot, cut the branch to one leaf. Do not do this after September.
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Eventually, your bonsai will get bigger. Think about it - five years of trimming at 2 branch nodes, twice a year or more. This means your tree will grow by 20 branch nodes in five years!
Some of this will be good for the tree. As the branches become smaller and more plentifull, your tree will look like it is a more mature tree. Some growth from the starting size of your bonsai is great for the look of the plant.
Some of this will not be good for the tree, the branching will make the plant look like a shrub. There a few types of trimming to be done to prevent ones tree from becoming a shrubby mess.
1: pluck clean the branches and twigs growing from the trunk or base of the larger branches. These will only conceal the beautiful trunk and branching of your bonsai.
2: as the tiny end twigs become more numerous, try to eliminate some of the larger branches. One can use the excess twigs from a still attached branch to hide the spot where the other branch has now vacated. The reason one does this is to clip off boring strait branches in an effort to encourage taper and movement; and to simplify the tree in a zen sort of way. This also can help to reduce the overall size of the tree.