Producing a precisely fitting mortise and tenon joint in one go using a saw is an art in itself. Perfection usually only comes with finishing by plane.
Commonly, this task is performed with a block plane.
The block plane is applied to the tenon and the mating surface is carefully shaped. There is a drawback, however: it is hardly possible to correct the angle that was previously produced by sawing. As a result, the tenon easily acquires a slightly conical form, causing a gap in the joint.
A simple trick will solve the problem:
The router plane, which is actually intended for shaping and smoothing grooves, is used here for trimming the tenon to the desired dimensions, parallel to the outer edge. A metal plane is suited best, but basically it is also possible to use a wooden one.
The procedure - step by step
Simply place the plane bottom on the planar outer side of the lengthwise moulding and fixate it by the pressure of one hand.
Now lower the blade to the lowest point of the tenon surface (or, alternatively, to a point previously calculated to result in the desired tenon thickness) and fixate the blade there.
The excessive tenon material is then removed with a twisting movement, but without pressure, at the second handle.
You should generally work from both sides to avoid chipping the edges. The same setting is then used from the other side to produce a precisely centred tenon. If the fit is still too tight, just remove more material according to requirements.