How does a previous heart attack represent itself on an ecg? Which wave and the form of the wave?

Q wave. Many, but not all old heart attacks show up as a q wave which is an initial significant downward deflection but this can also be seen in some normal variants. Reading ecgs can be tricky and significant training is needed to read them well.
Q wave persists. Q wave or qs wave persists and the st wave goes back to normal baseline.