Images of a small shooting board used for planeing braces. A low angle block plane works well. If the brace does not plane good, plane in a different direction. The brace shooting board has wood pegs at each end that act as stops for the brace.
Put a thin strip of wood when at the small end of the tapered brace when edge planeing to maintain a perpindicular brace when glued to top or back.
Braces do not have to very accurate but must not be too large or too small.