There are various methods to make a nice secondary mirror but one of them is this:
1) make a concave mirror with the same curvature as your secondary, often the grinding tool.
2) Finish it with the same conical constant as the desired convex secondary mirror.
3) Test this with traditional methods such as Foucault and/or Bath interferometry
4) Finish the real convex mirror with a new, plaster, tool.
5) Test the convex mirror with Newton rings.
6) Assemble the entire telescope.
For more accuracy:
7) Take a Dobson telescope of at least the size of the primary mirror
8) Put a fiber in the primary focus of the Dobson
9) Illuminate the fiber with a small LED.
10) Align the telescopes so that you will see a Airy disc in the center of the FOV of the Cassegrain
11) Do a Foucault and/or Ronchi test in the Cassegrain. It should be a null thus no shadows on the wavefront must be visible and/or Ronchi bands should be straight.
12) If they aren't, check you secondary.