kdc

Liteply up to one eighth inch and birch ply up to one sixteenth can be cut most accurately using a Stanley knife ( with a new blade ) and a straight edge. Make repeated light cuts along the same groove. If necessary turn over to the reverse side and complete the cut.
On one eighth or more birch ply make the groove with a knife and then cut with a saw such as a japanese saw ( pull saw ) or razor saw.
( japanese saws do not have a thick ' back ' like razor saws, dovetail saws etc, they cut on the pull stroke and therefore can be made very thin -excellent tool. )
Make sure the slot you cut is the same size as the part to go in it - if it is another piece of ply measure the actual ply because ' one eighth ' ply is sometimes only 3 millimetres.