Get on the air. Talk to people. Ask what they are using and how they like it. Either dip into your junk box or take a few dollars to your local Radio Shack store, and build the circuit in figure 1. This circuit simply takes the received audio, amplifies the signal, and sticks it into a status line of a serial port.
Software measures the time between the zero crossings to determine the frequency.
The software converts these tones into images on the screen. The circuit shown here is the bare minimum. There are many other minor variations on this theme using diodes for reverse voltage protection, capacitors for power filtering, and transformers for isolation. SSTV Home Page. Next Chapter of SSTV Handbook.