Working with hefty boards requires a fair amount of handwork unless you have equally beefy machinery in your shop, which I do not.
So, after I glued up the leg boards for my workbench, I had to flatten one edge of each with handplanes before running them through my power planer. I had already flattened the faces by hand.
Next I needed to square up one end of each leg with a handsaw in order to start laying out the through-dovetails and tenons.
I followed Chris Schwarz' model for handsawing big timber to length.
Scribe a cut line on all four sides of the legs. Start sawing the far corner of one face, keeping an eye on your line both on the top surface and adjacent face of the board. When your saw reaches the corner nearest you, flip the board forward.
You now have a starting point for your next cut. Continue to saw this way, flipping the board forward after each cut, until you've connected the kerfs on all four sides. You'll still need to saw out the middle, but your blade will follow the paths of the kerfs.
I was shocked at how well this method worked. If you lay out your lines carefully and saw straight, you can't miss.
The two 12/4 cherry boards that will become the benchtop sat in my shop for several weeks after I had flattened them by hand. This gave them time to get bent out of shape.
I'm not concerned at this point with the top of the bench, but the undersides must be flat so that they will rest evenly on the legs. This meant more quality time with my planes, winding sticks, and straight edge.