Well when it comes to a new lure design.
Answer to "no clue"
No clue = I just want a lure
New design = The best design that I think will work for what I am fishing for.
Been doing this for 30 years, that was a question I asked myself after the first several failures.
A new design needed a reason and a passion to work.
This year, it is a 7 plus pound Largemouth Bass in Michigan. On my bucket list, not a bucket mouth, just something I want to achieve before I pass the fishing rod.
Answer to "how do you do it" this is combined with "How do you do it", I use soft plastic designs as that has been the best in the MIdwest for me for 30 years, have em all from the Rapala, Rattle Trap, etc.....
Even Buck Perry's trolling system of lures. (That is when I started getting serious 25 years ago), read up and learn the father of structure fishing, as we know it today.
Symetrical parts,
Make one out of Sculpy fire the part and make a copy.
Exact "Symmetry", both parts exactly the same, I personally don't think is that important, Repala notes how you can "tune" a lure to fit your needs (or at least they used to have such information)
In My opinion a unique lure, is something the fish has not seen before.
That is why we make our own.
Easiest way to make a POP mold is to cut up soft plastics you like, super glue them back together, and make your POP mold.
Then if you like the lure and are a perfectionest (like used to be) and want to get both sides the same.
Put some Sculpey in the mold, squish it in good, fire the part, or a couple to use the same side for both sides of a new lure. Clean it up with little files or even a pocket knife (my favorite) glue it together and make your perfect lure
Best of Luck, good fishing
Hope this helps
Mark