How do you put nylon stirngs on an acoustic guitar if the string have little balls one end?

You should not put nylon strings on an acoustic guitar that is made to be played with steel strings. It's not just a simple procedure of threading the strings through and fixing the ball ends into place. Acoustic guitars built for steel strings are designed to handle much greater string tensions compared to nylon strings. Using nylon strings on such guitars will cause the bridge and internal bracing of the guitar to suffer a great deal of stress and possibly even lead to damage and the need for expensive repairs.

If you want to play with nylon strings, it's best to get an acoustic guitar that's specifically built for them. Such guitars will have a wider neck, different bridge and bracing construction to accommodate the lower string tension of nylon strings.