Look for shapes that will always let you eliminate other squares around them and/or place a tree. Here are a few examples to get you started, but there are lots of others as well.
Remember – some of these shapes might only appear after you eliminate other squares, decreasing the size of the original shape.
Remember that sometimes, even if you only know the general location of two trees, you might still be able to eliminate the rest of the row, e.g. here we can tell that there must be a tree in each of the circled locations, so we’d be able to eliminate the rest of the fourth row:
Sometimes, with double trees you’ll get situations like this, where the fact that you’ve already placed one tree in a row (or column) will help you place a tree in another row (or column). Since the third row already has one tree, we know there can only be one bright blue tree in the third row, meaning that the second light blue tree has to go in the circled space in the fourth row.
Sometimes you don’t even have to place the first tree. Here we can tell there must be a tree in one of the two red circled spaces, meaning that there can only be one dark green tree in the last column, therefore letting us place a dark green tree in the blue circled space.