Got Stripers is the man with the plan. He has a website that shows the basics.
In a nutshell, you buy the plastic from any of several sources (Bass Pro, Lurecraft, Netcraft, and several others, I believe) and then either buy or make a mold. Heat up the plastic in the microwave, add color and glitter and pour into the mold. Done!
Lurecraft has hundreds to choose from.
To make your own mold, you whittle the shape you want out of wood, and then buy some of that EXPENSIVE two-part silicone, and make your mold. Be SURE about the shape first!
I use soft plastics almost exclusively, so I have passed the break-even point, which is rare for fishing! The $100's I would have spent, I have anyway, but at least I have my own supply, not relying on anyone besides plastic suppliers.
Fortunately, so many people donate used plastic lures to me, I haven't bought plastic in a year, so it is working out OK.
For most it is probably more efficient to buy them from Got Stripers, and just stay at work an extra hour to pay for it, rather than spending the cash AND spending hours and hours making them yourself.
But I really enjoy the creativity involved in making everything from scratch.
TH
PS - Those "double" lures are still top producers when the big fish are around. I just got lucky with my first design!