Your question has several answers.

For some types of trees, you don’t—they won’t root from cuttings or roots.

Others can be propagated by:

• burying a side stem until it roots, then detaching it from the parent tree (layering)
• Taking a cutting and dipping it in rooting hormone and potting it—usually removing most of the leaves from the cutting to reduce transpiration
• Taking a piece of root and potting it up

Any of these could work for some tree species, but for some only one method will work.

Most conifers do not root from cuttings, but there are a few exceptions—some junipers and a few pines will root where a branch comes into contact with the ground.