My method (based on basses I've received and learned "WHAT NOT TO DO"...)
Leave the bass tuned to pitch. (as manufactures do)
The "shake test" is key; making sure the bass is not loose and firmly secured in its case.
Continuously video the packing and boxing.
(inexpensive phone tripod from Amazon)
If you have accessories, secure them in a padded envelope, or similar and make sure they cannot move.
If they're in a compartment, make sure the lid cannot come open (using bubble wrap to fill the void between the lid and top of the case).
Supplies.
- Bag of clean white painter rags. (Home Depot, Menards, Walmart, etc)
- Bubble Wrap. (small for in the case, large bubbles work well between the case and box.)
- Masking tape and or "stretch-wrap".
I Basically "mummify" the bass, starting at the headstock, with the white rags, moving down the neck, before placing it in the case.
wrap the headstock - secure (I use the stretch wrap, using light pressure.)
Next, leaving the strings tuned to pitch, I put rags
between the the neck and strings**, and then wrap rags around the neck and over the strings, and secure with stretch wrap
Now I put the bass in the case, placing bubble wrap under / over the headstock & neck, (both protected by the rags) making sure the lid closes - just making a little contact, but without pressure.
Now I cover what's rest of the body, and roll up rags around the perimeter of the body between the case - wherever there is slack or movement.
The bass should not be able to slide at all in the case.
Finally, fill the rest of the voids with bubble wrap (which never directly touches the bass itself) - all while opening / closing / shaking the case as you go.
Now "test fit" the case into the box; I put an extra layer of bubble wrap at the bottom of the box. (or tape it to the bottom of the case)
Bubble wrap the bottom 1/2 of the case, using masking tape to hold it in place. Fit it (& refit it) until there's enough bubble wrap until it's snug.
Once you have a good fit, cover the top 1/2 of the case with bubble wrap, but it doesn't have to be snug - it's easy to put more bubble wrap in the top 1/2 of the box once the case is in the box. Once the case is entirely in the box, enough Bubble wrap to the top 1/2 so it's snug and seal it up.
(I prefer using the brown carton-sealing tape 1st, then good quality 3m packaging tape.
**(I've once received a feretless with some pretty good marks on the fingerboard. The box likely to a blow or got dropped, causing the strings to slap against the fingerboard. Fretless or Fretted, that can't be good, and the rags are an easy solution)
Oh - and always UPS / Pirateship