Here is a blue inexpensive blue cello with the worst crack. If it was a decent instrument you would use the following steps:
take the top or back off
make a mold
Glue the crack together
carve out the area for the patch
Chalk fit the patch
Glue the patch in
Make the patch the same thickness as the back
Cleat the rest of the crack
Thin the cleats
Glue the top or back onto the ribs
Seal the outside of the crack
For this cello I would not do all that work, but I did want to entertain myself and see if I could fix it without all those steps.It was buldging out quite a bit, so I needed to get it back to a normal curvature. I needed to clamp it to somthing, These are the steps:
Make a clamping jig that is suspended just over the top and is clamped onto the ribs.
Then make a clear plastic mold.
Glue and clamp the crack.
After a couple of days when I had time, I made a little oval reenforcement that the sound post would sit on.
To glue that in I made a smaller plastic mold then while the oval piece was still wet I used a couple of drops of glue to hold it onto the plastic mold.
Put glue on the oval piece and dropped it into the cello.
Then moved it into place
I used an adjustable soundpost jack to hold it down.
The next day I popped the plastic clamping piece off the oval.
Then when t was all finished, just a little clear touchup on the crack to seal it.