It really depends where it is going.
Sometimes dimension restrictions make it prudent to minimise the dimensions even more, though that is unusual.
Most couriers within the UK will take a full cycle box for a decent rate.
Anyone about to pack a cycle should get hold of some proper packing tape and leave nothing loose or unpadded. I use the cheap LIDL gaffa tape.
Take rear mech off if in doubt, there is a good reason we use replaceable dropouts. Derail the chain, and wrap it with cardboard tubing.
Cycle boxes will allow the seat to remain in situ, just lower it and make sure it is tight.
You can leave the bars on, where you don't have a face off stem you can just take one grip off and slide everything to that end. Other wise wrap the whole thing then take it off and strap it onto the head.
Dislocate the brakes if needed. Loose off gear cables.
Whenever you see old expanded pipe insulation in a skip grab it, it is invaluable for packing frames.
Make some ritual markings inside the box for good fortune.
Clearly mark the box if needed, this way up and fragile, do not crush.
Cross fingers, but never send anything insured for less than 150%.
The doctor diagnosed me with hereditary diarrhoea. I can't understand, I wear a kilt.