If you need to adjust the size of a mortice then the temptation is to simply have a go with a chisel. If you're like me, the end result is never 90 degrees to the face.
But clamping a pice of scrap as a guide like so
means you have a nice flat support for your chisel.
I find that gently paring away in an arc and little by little is the best way to trim the mortice as opposed to a full-frontal assault. Skew chisels make this a doddle.