So... first, I start off with the trunk and branches. For the trunk, I use a 2x2 format and build it upwards, making sure the logs are almost never the same height... if that makes sense? xD For example, at the base, one is always taller/shorter than the other. As for the branches, I try to split the tree into two main branches. From there, I make smaller, sub branches going off in all different directions.
Additionally... I like to delete the corner of the tallest trunk... block? xD Hopefully this screenshot makes everything come together and make sense... hopefully
Then, I make small spheres around each branch tip. If the branch is too close, you can always slightly move the bunch of leaves. To make the tree look more airy than dense, simply just remove a few leaf blocks from the final product.
After filling in the tips of the branches with the main leaves, I go in with another type of leaf block to give it texture. For example, some good and basic combinations would be oak with jungle and oak with birch. For this little tutorial I used oak with jungle.
After that, you're done! :D Now, just decorate it! >w<
If you'd like, I could redo it and give more in-depth instructions on how I usually do my trees ^-^
