All of our knobs have the screw pre-mounted into the knob and install from the front of the door or drawer. The knob is secured on the back side of the cabinet door with a washer and nut. Some of our porcelain knobs have screw size options, so check the product listing for options.
To customize the length of the screws, handymen use a variety of tools to cut the screws. If you know anyone who does crafts, likely they have a Dremel tool cutting wheel that does a very clean cut. A hack saw will work but perhaps not as clean a cut. Professional installers have other cutting tools available to them.
To create a flush mount on the back of the cabinet door or drawer, another handy tool is electric screwdriver bit called a counter sync bit. It drills a very small half moon shaped notch into the back of a cabinet door just large enough to allow the nut to be wound back onto the screw. The flush mount not only looks cool, but also prevents any accidental encounters with the end of the screw.
Good luck with your installation. If you need any further information, don't hesitate to contact me. I'll be happy to help you.