Downsampling an Oversampled packet I have series of amplitudes of a signal recieved at center frequency of 433.9MHz, sampling at 250000 Hz. I can see the information received is the packet I am after, just sampled too high. I believe the transmitter transmits at 3918bps. What is the best method to downsample from 250000 Hz to 3918 bps?

I have tried using a butterworth filter, which doesn't seem to work and I don't understand why. I've tried picking every 63 samples (because 250000/3918 = ~63), but didn't seem to work either. Please bare in mind I am a bit of a newbie regarding DSP.
Based on your information a packet length is about 77.2 millisecond. With 3918 bps, each packet contains about 302 bits. Frequency is shifting about every 0.255 millisecond. you get about 256 samples per bit, so 128 point FFT seems to be correct. I assume you have a real signal, so you need to evaluate only 64 bins. If you are synchronized properly, each two consecutive FFTs will provide you with one frequency, switching at the other. If you are not synchronized with the start of the first bit, only every other bin will provide you with a nice single frequency (assuming that you do not have noise and spurious signals)