There are three basic shapes of tree that I have tried to simplify for you in the video, an umbrella, a lampshade and a teardrop
An umbrella tree should give you the impression of weeping willows and oak trees, The lampshade, elms and sycamores and the teardrop, poplars and Christmas trees.
If you look at the diagram I have painted trees without leaves to show you how they might look in winter if they are a deciduous tree and loose their leaves.
If your tree has leaves (foliage) you must paint the foliage first or the trunk will bleed through, take a look at the video and you will see that I paint the foliage first leaving gaps for the tree trunk and branches to be painted in.
Be kind to birds and always leave gaps in your foliage so that they might be able to fly through the tree without breaking their necks.
Remember also that as a tree grows from the ground up you must paint your trunk from the ground up so that the trunk will always be thicker than the branches at the top of the tree.