Hi,
I'm making a housing for an electronic musical instrument - basically
a box, sloping down from back to front, with a metal faceplate on top
instead of a lid. I'm using black walnut.
I want to use what I think are called box joints or finger joints to
join the four side pieces. I've marked up my pieces, and I did the
same on some scraps and tried cutting joints as a test. Now I'm doing
this by hand with a basic dovetail saw, as I don't have a table saw,
and I don't see how to make sure that I'm cutting straight, and
perpendicular to the top edge.

In my test, I tried clamping two pieces together, offsetting them by
the width of the saw blade, and cutting into the waste sections, so
doing both pieces at the same time. The cuts were often not straight
enough.

Two problems:
1. Marking up. The vertical edge of my piece of wood is cut by hand,
so it won't always be exactly square to the bottom horizontal edge. So
if I use a 90 degree square from the vertical edge to mark up the
horizontal join cuts, they won't always be exactly parallel with
bottom edge.
2. It's difficult to cut in a straight line!

Any tips? Should I be using a saw guide of some kind? I have a feeling
I will get better results just cutting one piece of wood at a time,
instead of trying the two clamped together trick.