How to flatten handsaw cut edge?
Cutting a miter like that is best done on a table saw with a crosscut sled, or with a well tuned miter saw, or (failing either of those options) rough cutting the miter with a hand or circular saw and then dressing them with a hand plane or large disk sander. Given the dimensions your best option at this point is to use what's called a "donkey ear shooting board" (shown here) with an appropriate hand plane on a good solid workbench. For best results you will need a fairly sharp low-angle jack plane to perform this operation but it can be done with a standard jack or jointer plane if they are very well tuned. Best of luck!