If you write the above title into google you will find the most amazing wealth of utube videos and other blogs demonstrating how to paint trees in watercolour. I love this digital age of plenty that we live in where we can, usually share freely in the talent and ideas of others to hone and develop our own skills
One utube video that I have used recently was this one
I find the best way to have a go is to find what materials you have to hand that are similar to what is suggested and then sit to watch the clip. Then I fix the clip onto a still of a section of the process and have a go myself, always using the biggest watercolour brush I am brave enough for. The image on the right below is my attempt at the utube link approach and you can see immediately that I have made a typical beginner's error and not gone dark enough with the darks; something you could usefully remember if you fancy a shot at it. It works well because it only uses two colours, ultramarine and raw sienna. The image on the left is a spring picture I did of the woods near St Hilaire.