Actually sighting in a sightless cannon isn't that difficult once you learn the skill.
Just look down the barrel adjust windage and elevation and fire. Here is the first target I ever fired using this technique.
Okay so it isn't that simple. Here's a little more explanation.
In order to aim the tube use the alignment of the front muzzle ring with the rear cascabel ring. Align the two rings so their 12 o'clock positions align with your aim point for elevation and the 9 o'clock or 3 o'clock position points to your windage aim point.
Here's a better illustration of what I am saying.
Lets say just for discussion you have never fired your gun before and you need a starting point. 6 o'clock on the black is always a good aim point to start so align the the 12 o'clock position of your muzzle and cascabel with the 6 o'clock position on the bull.
Now look at the 3 o'clock position of your muzzle and cascabel and align them with vertical center-line of your target.
Now verify you haven't moved your elevation and fire.
To adjust, look at how far below horizontal to the bullseye your shot hit. You new elevation aim point is now the same distance above 6 o'clock that your shot hit below the bull.
For windage your new aim pot is the same distance to the left of the center line as you your shot hit to the right.
Recheck windage and make sure you didn't move it and fire
Extreme accuracy is possible using this sighting method. Of course you will have to have a good load also, but that is a whole other discussion.