Spark plug boots are made of rubber, are flexible and are non-electrical conductors. These are important aspects because the wire that attaches to a spark plug, when not covered by a boot, will follow the path of least resistance and spark on the iron mower deck instead of inside the mower cylinder. This will cause your engine to misfire and run poorly. If your spark plug boot is torn and shredded, it is not worth repairing, and you're better off to get a new one to slide onto the wire. But if it is just cracked a bit, consider a cost effective way to repair it as good as new.
Cut a piece of nylon screen no wider than the boot itself with your scissors. You should just be able to wrap it once around the boot. It doesn't have to be perfect as long as it will wrap around one full time.
Clean the boot of all oil and grease by spraying it with a solvent and wiping it off with an old rag. Let it dry completely.
Smear a light coat of silicone onto the boot, covering it completely.
Wrap the nylon screen around the boot, making sure it comes into full contact with the silicone.
Smear another light coating of silicone over the top of the nylon screening. Allow to dry overnight.
Check the boot the next day. It will be solid, yet flexible, and 100 per cent repaired.
If you get any silicone onto your fingers, use the same solvent you cleaned the boot with to clean your fingers.