Best way to do that is to take a sacrificial piece of thin plywood
place the unevenly shaped block on the ply- and make some wedges that go in and support/cradle the block- then run maybe one or two short screws through the plywood sled into the block, at each end- in the area that might get trimmed off when the shafts are finished.
then run it through the saw with the sled running up against the fence.
once i had a flat face, i would take the sled off, now you can cut boards to your hearts content.
having done this before- cutting bow lams from odd chunks of wood, i would, once a flat face has been established, flip the block 90* re-attach to the sled, and cut another face at 90* to the first one.
you dont need this second face to cut boards, but will find it waaay easier to cut the square blanks if you have a straight edge on the boards.
just make sure that when you do these square cuts, that you are following the grain perfectly- to ensure that your finished arrows have minimal grain runout!
the initial square face cuts dictate how the rest of the operation will go.
oh, and dont cut the screws
