How do you get spark plugs to thread when they keep turning?

Ok allowing you have the spark plug in the hole but the threads won't grip: You have stripped the threads. This is easy to do.

Go and buy a special for spark plugs thread repair kit. Follow the directions in the Kit.

If there is not one available for your application you probably will have to remove the cylinder head and take it to a machine shop to be repaired.

Very rarely will a machinist be able to repair on the car.

Big job, Expensive and difficult.

Good Luck