How to make realistic trees?

If you do not want to buy pre-made trees, which these days look very realistic, you can get tree armatures which are basically the trunk and branches. They come in various sizes from ¾ of an inch up to 7 inches (from 2cm to about 17cm). Simply bend the branches into shape and put onto the supplied bases and paint.

Do not make the mistake of painting them completely brown as tree trunks and branches are a mixture or colours (browns, green, yellow with the dominant colour being grey).

Then add some tree foliage (readily available from your model shop): add some PVA glue to the branches and foliage and apply. When dry add some hairspray over the top which will give extra hold and help prevent the foliage falling off over time or excessive handling.