There are a few things that may help.
The cheapest route would be to trace the higes in their location with a pencil.
Then cut line with a sharp knife. When you have the outline cut with the knife,
Take a sharp chisel and peel away the center. Use a hinge to check the depth of the cut.
Don't dig too deep. Just whittle away what you have to to get the depth you need.
You should be able to push the chisel with your hands, try to resist the urge to use a hammer.
Oh, and predrill the screw holes. The frame will likely split if you don't.
Take your time with this. It is very easy to gouge the door frame past the edge of the outline.
You may have to score the line several times with the knife.