Get to grips with the various selection tools, then tackle intricate extraction tasks with Photoshop’s Refine Edge
Go to Select>Color Range and click on the sky. The selection preview will show white in places that are similar in colour to the area clicked on. Max out the Fuzziness slider to increase the selection, then click OK.
Use the Quick Selection tool to finish selecting everything outside of the tree as well as any gaps within. Click and drag to add to the selection. The left and right brackets resize the brush. Opt/Alt-click and drag to remove areas.
It will be a rough selection at this point. To make it accurate, go to Select>Refine Edge. This dialog box contains lots of useful ways to perfect the selection. Use the Zoom and Hand tools to zoom and pan your image.
The Refine Radius tool improves selections by adding fine detail that would be incredibly hard to capture manually. Paint around the edges of the tree and in between gaps. Photoshop will think for a second, then magically refine the selection!
Check the Decontaminate Colors option to remove stubborn colour fringes. These are replaced with the colour of neighbouring pixels. Use the Amount slider to adjust the amount of colour fringe to remove.