How can I prepare an uneven beam to be wrapped with wood?
With a helper, pull a builder's line (dryline) from one end to the other along a bottom corner of the beam. Find a good compromise that's just outside all the curves and humps. Fasten the line to the walls at each end and make sure it's very tight. 
Check that you can plumb up from the line at all points along the beam without encroaching on the beam. If not, move the line outward as necessary to clear the beam. 
Check the line for level and square with the room. Adjust as needed. 
Set another line on the other side of the beam parallel to and level with the first, again just outside the beam's extremities. 
Plumb up from both lines at the four ends and mark the ceiling. Snap chalk lines along the ceiling to establish straight lines at the top. Pull the chalkline perpendicular to the ceiling when you snap it to prevent curvature due to a non-flat surface.
At 16" intervals, fit and install shims, cut from two-by lumber, around the beam that come just shy of both lines and the ceiling marks. Don't touch the line or you'll push it out. This has a cumulative effect and will result in curves. Each set of shims should result in a plumb, level, square panel mounting base.
Install your panels over the shims as though they were a framed wall.