The RF modulator has a row of switches which have to be set to a specific sequence in order to select the UHF channel (21 - 69) that you want it to use.
Your TV set looks for broadcast channels in that range and the idea is that your TV set thinks the RF modulator channel is simply another channel that it's receiving from the aerial.
You must find an unused channel and set your RF modulator to that number. If you choose a channel that is already carrying a broadcast then you'll see interference on the picture. Choose a different one.