Bevel a 2x6 long ways
I voted to close this since it's really a construction question and belongs in the DIY site, but the best way to do this is going to be shimming up one side and running the board through a planer.
Basically you'll draw your angle on the endgrain, get a piece of MDF wider and longer than your work piece, then use framing shims to get your line parallel to the sheet of MDF.  Tack/screw the shims in place and then tack stops on all the sides of the board to keep it from moving on the MDF.  Make sure none of your brad nails/screws will contact the planer blades (or poke through the MDF to hit the planer bed).  Then run it through till you hit your line.